Acid-treated dried lemna minor as an adsorbent for the removal of copper and lead from an aqueous solution

Table of Contents

ACKNOWLEDGMENT

List of Tables

List of figures

List of Acronyms

Abstract

1. INTRODUCTION
1.1 Problem Statement
1.2 Research Questions
1.3 Objective
1.3.1 General Objective
1.3.2 Specific Objectives
1.4 Significance Of The Study
1.5 Scope Of The Study

2. Literature Review
2.1 Health and Environmental Effects Of Heavy Metals
2.1.1 Health Effects Of Copper
2.1.2 Health Effects Of Lead
2.2 conventional methods
2.3 Bio sorption of heavy metals
2.3.1 Merits Of Biosorption
2.3.2 Biosorbent Materials
2.3.3 Gaps In The Literature
2.4 Characteristics Of Lemna Minor
2.4.1 Previous study of Biosorption on to lemna minor
2.5 Adsorption Mechanism
2.6 Factors Affecting Biosorption
2.7 Adsorption Equilibrium Model
2.7.1 Adsorption Isotherms
2.7.2Adsorption Kinetics Models

3. MATERIALS AND METHODS
3.1 Equipment And Chemicals
3.2 Experimental Methods
3.2.1 Preparation Of Acid Treated Dried Powder Of Lemna Minor
3.3 Characterization Of Lemna Minor
3.3.1 Proximate Analysis Of Lemna Minor
3.3.2. Fourier Transform Infrared Spectroscopy (FTIR) Analysis Of Lemna Minor
3.3.3 X ray Diffraction Spectroscopy Analysis Of Lemna Minor
3.3.4 Point Of Zero Charge
3.3.5 Specific Surface Area
3.4 Preparation Of Aqueous Solution
3.5 Batch Adsorption Experiments
3.5.1 Effect Of pH On Metal Ion Adsorption
3.5.2Effect Of Initial Metal Ion Concentration
3.5.3 Effect Of Adsorbent Dose On Metal Ion Adsorption
3.5.4 Thermodynamics Studies
3.6 Adsorption Isotherms
3.7 Kinetic Studies On Metal Ion Adsorption
3.8 Experimental Design For Biosorption Study Of Copper And Lead
3.9 Desorption Of The Metals

4. Result and Discussion
4.1Proximate Analysis
4.1.1 Point Of Zero Charge
4.2 FTIR Analysis
4.3 XRD Analysis
4.5 Surface Area Analysis
4.6 Effect Of pH
4.7 Effect Of Initial Metal Concentration
4.8 Effect Of Adsorbent Dose
4.9 Temperature Effects
4.10 Adsorption Equilibrium Studies
4.11Thermodynamics Studies
4.12 Kinetic Studies
4.13 Desorption Study
4.14 Data Analysis Using Design Expert 6.0.8
4.14.1Model Adequacy Check
4.14.2 Single And Interaction Effects
4.15 Optimization Of Percentage Removal Of Metal Ions

5. Conclusion And Recommendation
5.1 Conclusion
5.2 Recommendation

Reference

APPENDIXES
APPENDIX A: standard calibration curves of copper (II) and lead (II)
APPENDIX B Fourier Transform Infrared Spectroscopy (FTIR) Correlation chart (source, https://www.chem.wisc.edu>handouts)
APPENDIX C: Photos Taken At Different Stages Of Laboratory Works
APPENDIX D: Results For Different Isotherm Values
APPENDIX E: Analysis Result Values For Kinetic Parameters
APPENDIX F: Analysis Results Of Temperature Effects On Percentage Removal

ACKNOWLEDGMENT

First of all, I would like to thank my Almighty God for giving me the strength and patience in accomplishing my thesis. Words fail to express my gratitude to “KAAD” in country scholarship program for awarding me the scholarship. My next gratitude goes to my Advisor Dr. Eng. Abubeker Yimam for his valuable Guidance and fruitful comments from inception to completion of the research. I would like to extend my acknowledgment to chemical eningering technical assistants especially Mr. Hintsa and Mr. Aklilu for their support during the laboratory works.

Last but not least, I am very much pleased to express my heart felt gratitude to my family and to my mother W/ro Wahid mesfin for her untired support, encouragement and for giving me the meaning of life.

List of Tables

Table 2.1 United States environmental protection agency EPA of electroplating effluent standards

Table 2.2 Comparison of bio sorption maximum capacity of Cu (II) on different biosorbents

Table 3.1 Experimental factors and levels

Table 4.1a Untreated lemna minor matched phases of XRD results

Table 4.1b Acid treated lemna minor matched phases of XRD results

Table 4.2 Languimur separation factor RL for copper and lead

Table 4.3 Summary of equilibrium parameter values of Pb+2 and Cu+2

Table 4.4 Pseudo first order, pseudo second order and intraparticle diffusion parameter values

Table 4.5 Desorption of Copper and lead by using a 100 ml of 0.1 M HCl

Table 4.6a Analysis of variance (ANOVA) for Copper (II)

Table 4.6b Analysis of variance (ANOVA) for Lead (II)

Table 4.7 Model adequacy parameters for copper and lead biosorption

List of figures

Figure 2.1 schematic diagram of biosorption process of metal ions

Figure 2.2 lemna minor leaves

Figure 2.3 scanning electron microscopy image of modified lemna minor before and after use as an adsorbent

Figure 3.1 schematic representation of batch biosorption process

Figure 4.1 proximate analysis of raw and acid treated lemna minor

Figure 4.2 pH of point of zero charge of lemna minor

Figure 4.3 FTIR spectra of untreated, acid treated, copper loaded and lead loaded adsorbent

Figure 4.4 XRD patern of untreated and acid treated lemna minor

Figure 4.5 plot of percent removal of Cu +2 and Pb +2 Versus pH on ATLM

Figure 4.6 plot of percent removal of Cu +2 and Pb +2 versus initial metal ion concentration

Figure 4.7 plot of percent removal of Cu+2 and Pb+2 versus adsorbent dose

Figure 4.8 a Plot of adsorption capacity of Cu +2 versus initial metal ion concentration

Figure 4.8 b Plot of percent removal of Cu+2 versus initial metal concentration

Figure 4.8c Plot of adsorption capacity of lead versus initial metal concentration

Figure 4.9a, b Languimur isotherm of copper (II) and lead (II)

Figure 4.9c, d Freundlich isotherm of copper (II) and lead (II)

Figure 4.9e, f Temkin isotherm of Copper (II) and lead (II)

Figure 4.10 Van’t hoff plot for adsorption of Cu and Pb on to ATLM

Figure 4.11 Kinetic parameter values of Lead (II)

Figure 4.12 Kinetic parameter values of Copper (II)

Figure 4.13 Percent desorption of Cu+2 and Pb+2 with 0.1M HCl

Figure 4.14 Normal probability plot for Lead (II) and Copper (II)

Figure 4.15 Predicted versus actual experimental values for biosorption Lead (II) and Copper (II)

Figure 4.16 Single interaction effects of initial metal concentration versus adsorbent dose of Lead (II) and Copper (II)

Figure 4.17 Single interaction effects of pH versus adsorbent Dose of Lead (II) and Copper (II)

Figure 4.18 Single interaction effects of pH versus initial metal concentration of Lead (II) and Copper (II)

List of Acronyms

illustration not visible in this excerpt

Abstract

The release of waste water with heavy metals of Cu, Pb, Zn, Cd etc... Particularly in areas with expansion of industries can pose a significant threat to human health and environment due to their toxicity. This paper deals with bio sorption of heavy metals (copper and lead) using acid treated dried lemna minor powder. The adsorbent after treated with hydrochloric acid was dried and ground to a particle size range of (820-850) µm. Characterization of the adsorbent such as proximate analysis, surface charge, FTIR spectroscopy, surface area and XRD was done prior to bio sorption process. A batch adsorption experiment was carried out using central composite design expert 6.0.8 and the effect of adsorption parameters: pH, initial metal concentration, and adsorbent dose was studied. The result indicated that maximum percentage removal (92.45%) for Lead (II) was obtained at pH = 11. Similarly, 88.4 % removal was found for Copper (II) at pH = 9. Temperature effects showed that the adsorption process was endothermic with positive enthalpy and negative free Gibbs energy values. The Removal efficiency of lemna minor increased with increase in temperature. Data’s from the adsorption were analyzed with Temkin, Langmuir and Freundlich isotherm models. Kinetic models such as intraparticle diffusion, pseudo first order and pseudo second order were used to analyze adsorption kinetics. Desorption of Cu+2 and Pb+2 were carried out by using a 100ml of 0.1M HCl in to 250 volumetric flasks with 0.1 g of 43.86 ppm of copper and 42.38 ppm of Lead loaded sun dried lemna minor. The percent desorption increased with time for both metals and maximum desorption observed at 60-minute contact time with 80% for Lead and 72.2% for Copper

Key terms: Heavy metals, Lemna minor, Biosorption, Desorption, Percent removal, adsorption capacity

1. INTRODUCTION

At present, Water pollution as a result of industrial development has become a common problem. With increasing generation of heavy metal bearing effluents from various industries, many aquatic environments are prone to metal concentrations which exceed water quality criteria designed to protect the environment. Heavy metal containing wastewaters are mainly released from electroplating, metal finishing, metallurgical, chemical manufacturing, mining and battery manufacturing in considerable amounts (Fu F et al, 2011).

Heavy metals are chemical elements with a specific gravity that is at least 5 times the specific gravity of water and are toxic or poisonous even at low concentrations. Some well-known toxic metallic elements are arsenic, iron, chromium, cadmium, Copper, lead, and mercury. Due to expansion of industrial activities in developing country, the levels of heavy metals in water systems have greatly increased. They can easily enter the food chain because of their high solubility in water. These heavy metals being non-biodegradable can pose a serious problem, with human health concerns and environmental consequences (Hawari and Mulligan, 2006).

Heavy metal toxicity can result in damaged or reduced mental and central nervous function, lower energy levels, and damage to blood composition, lungs, kidneys, liver, and other vital organs. Long-term exposure may result in slowly progressing physical, muscular, and neurological degenerative processes that mimic Alzheimer's disease, Parkinson's disease, muscular dystrophy, and multiple sclerosis. Allergies are common, and repeated long-term contact with some metals can cause cancer (Uluozlu O et al, 2008). Stringent policies with regard to the metal discharges are being enforced particularly in industrialized countries. Due to their high toxicity, industrial wastewaters containing heavy metals are strictly regulated and must be treated before being discharged in to the environment. This situation will require the development of simple and low-cost methods for the removal of heavy metals from waste water prior to discharge. The currently employed technologies for removal of heavy metals from industrial effluents appear to be inadequate and expensive. They often cause secondary problems with metal bearing sludge’s. It is therefore essential to develop alternative technologies to treat metal bearing effluents (Ushakumary, 2013).

Previous studies have showed that the adsorption onto activated carbon is a reliable and significantly effective technique for removal of heavy metals; however, the high cost of activated carbon limits its use as an adsorbent. (Balarak D et al, 2016; Dizge et al, 2008; Cengiz, S et al,2008; Lata et al., 2014; Batzias et al, 2007). Recently, various materials such as fly ash, wheat straw, apple pomace, fungus, and orange peels, soy meal hull, eggshell membrane and etc has been applied to develop low-cost and effective adsorbent (Zazouli et al, 2014; Doulati et al, 2008; Arami et al, 2006). The Lemna minor is one of wide-spread aquatic plant that belong to duckweed species with special characteristics including rapid growth, high nutritional value, and high-water purification capacities have been used to remove the pollutant from water and wastewater. Several studies to remove the heavy metal, dyes and etc by the Lemna minor have been conducted by the scientists (Balarak et al, 2015, Ge et al, 2012; Alvarado et al, 2008; Uysal et al, 2013). Therefore, the aim of present study was to assess the capacity of Lemna minor to remove the copper (II) and lead (II) and the effect of parameters such as pH, adsorbent dose and initial metal concentration was investigated.

This thesis contains a total of five chapters. The first chapter explains general background of the study, objective, research questions, significance and scope of the research. Chapter two deals with the literature review where similar related papers were discussed from which research gaps were identified. Materials and methods and overall methodology followed were discussed under chapter three demonstrating what has been done. Chapter four is all about result and discussion with the help of laboratory datas. Finally, Conclusion of the works done and recommendations for future work were included on chapter five.

1.1 Problem Statement

Heavy metals are chemical elements with a specific gravity that is at least 5 times the specific gravity of water and are toxic or poisonous even at low concentrations (Ushakumary, 2013). With increasing generation of heavy metals from industrial activities, many aquatic environments face metal concentrations that exceed water quality criteria designed to protect the environment (Abdelwahab, 2007). They are highly dispersed in a wide variety of economically important minerals. They are released to the environment during mineral extraction process. Therefore, mining activities are the first anthropogenic source of heavy metals. These heavy metals have potential health risks associated with metal uptake via food chain, dermal absorption or inhaling. High levels of exposure to heavy metals have been proved to cause cancer, organ damage, joint diseases, and in extreme cases, death (Jarup, 2003). Several processes exist for removing dissolved heavy metals, including, ion exchange, precipitation, ultrafiltration, reverse osmosis, electro dialysis and activated carbon (Joshi, 2017). Many of these approaches demand high energy, high cost, advanced operational requirements, result in large amounts of sludge requiring treatment or difficult to treat and be disposed of in an environmentally sound manner, or do not enable recovery of metals or material.

From this point of view, there is a need to look for a low cost, simple, and efficient heavy metal removal technique. A number of biosorption studies were carried out by various researchers as biosorption is quite popular for its simplicity and efficiency (Rana et al, 2014). Bio sorption has potential advantages over conventional methods such as low cost, high efficiency, less use of chemicals, recovery of metals and reuse of the adsorbate, and minimum waste (Omran et al, 2015). Several studies to remove heavy metals, dyes etc by the Lemna minor have been conducted by researchers (Balarak et al, 2015, Ge et al, 2012; Alvarado et al., 2008; Uysal et al., 2013). The aim of the present study was to evaluate the capacity of acid treated (HCl) Lemna minor to remove copper (II) and lead (II) from aqueous solution and to investigate the effect of parameters such as pH, adsorbent dose and initial metal concentration on its removal efficiency. The purpose of treating the bio sorbent with acid is to enhance its adsorption capacity

1.2 Research Questions

- Are there sustainable and available bio adsorbents such as lemna minor (duckweed) that can be used for the removal of heavy metals?
- Can the emerging bio adsorbents actually replace activated carbon which is very expensive adsorbent common today?
- What is the optimum Operating parameters for biosorption of metal ions under batch studies

1.3 Objective

1.3.1 General Objective

The general objective of the thesis was investigation of acid treated lemna minor as an adsorbent for removal of Cu (II) and Pb (II) from aqueous solution.

1.3.2 Specific Objectives

- To prepare and characterize lemna minor for bio sorption process
- To determine specific surface area of the bio adsorbent
- To carry out batch adsorption of heavy metal ions particularly Pb+2 and Cu+2
- To determine optimum values of adsorption parameters such as pH, initial metal ion concentrations and adsorbent dose on the efficiency and removal capacity of adsorbent
- To analyze the data obtained from the adsorption experiment using the sorption isotherm
- To determine the adsorption kinetics of the adsorption process
- To desorb the used adsorbent for possible metal recovery

1.4 Significance Of The Study

Up on completion of this research, it can contribute to the future applications of adsorption by providing information about bio sorption of heavy metals on acid treated lemna minor. Recently, bio sorption techniques for removal of heavy metals have gained attention. There are several applications of bio sorption such as metal plating and metal finishing operations, mining and ore processing operations,metal processing (waste recovery), battery and accumulator manufacturing operations, thermal power generation (coal-fired plants in particular), and nuclear power generation.Thus, bio sorption is a good treatment method in comparison with other techniques and it has major advantages including low cost, high efficiency, minimization of chemical and or biological sludge, no additional nutrient requirement, and regeneration of bio sorbent and possibility of metal recovery (Kratochvil and Volesky, 1998). After investigation of the research, the result can pave the way towards the improvement heavy metal removal in our country.

1.5 Scope of The Study

This research generally covers bio sorption and desorption of heavy metals (Cu+2, Pb+2) ions from aqueous solution on to acid treated lemna minor powder. It included preparation and characterization of the adsorbent by using proximate analysis and standard testing procedures (ASTM), determining specific surface area of adsorbent, determining functional groups of the adsorbent by using FTIR, and crystalline structure of the adsorbent with XRD, measuring initial and final concentration using UV/vis spectrophotometer. It also covers batch adsorption and desorption experiments on various factors including biosorbent dosage, initial metal concentration and pH. Result and discussion were made following the laboratory datas.

2. LITURATURE REVIEW

Water is essential for the existence of all life forms. In addition to household uses, water is vital For agriculture, industry, fishery and tourism. Increasing population, urbanization and industrialization has led to the decreased availability of water. The quality of water used is also being deteriorated as it is getting more and more polluted. Water pollution may be defined as the contamination of streams, lakes, seas, underground water or oceans by substances, which are harmful for living beings. A large amount of water is discharged back after domestic and industrial usage. This is contaminated with domestic waste and industrial effluents. When this contamination reaches beyond certain allowed concentrations, it is called pollution and the contaminants are called pollutants.

2.1 Health and Environmental Effects Of Heavy Metals

Heavy metals are chemical elements which have a specific gravity at least 5 times the specific gravity of water and is toxic or poisonous even at low concentrations. Some well-known toxic metallic elements are arsenic, iron, Chromium, cadmium, lead, mercury, copper, nickel, lead, etc. Heavy metals are highly dispersed in a wide variety of economically important minerals. They are released to the environment during mineral extraction process. Therefore, mining activities are considered as the primary anthropogenic source of heavy metals. Heavy metal ions are discharged into water system from various industrial activities such as electroplating industries, electronic equipment manufacturing, and chemical processing plants. Due to rapid development of industrial activities, the levels of heavy metals in water systems have substantially increased.

Heavy metals can easily enter the food chain because of their high solubility in water. Cadmium, copper, chromium, lead and zinc are extremely toxic heavy metals of widespread use in many industries (ushakumary, 2013).

Heavy metal pollution is affecting the society with human health concerns and ecological consequences. It is essential to remove heavy metals from industrial waste waters. Numerous waste biomass sources are available in nature in which adsorption properties have been reported e.g rice husk, saw dust, tea and coffee waste, orange peel peanut shells, activated carbon, dry tree leaves and barks (Asma et al, 2005; Ferda and Selen, 2012; Kishore et al, 2008; Nuria et al, 2010). Adsorption of heavy metal ions occur as a result of physicochemical interaction, mainly ion exchange or complex formation between metal ions and the functional groups present on the cell surface.

2.1.1 Health Effects Of Copper

Environmental contamination due to copper is caused by mining, printed circuits, metallurgical, fiber production, pipe corrosion and metal plating industries. The other major industries discharging copper in their effluents are paper, pulp, petroleum refining and wood preserving. Agricultural sources such as fertilizers, fungicidal sprays and animal wastes, also lead to water pollution due to copper. Copper may be found as a contaminant in food, especially shell fish, liver, mushrooms, nuts and chocolates. Any packaging container using copper material may contaminate the product such as food, water and drink. Copper has been reported to cause neurotoxicity commonly known as “Wilson’s disease” due to deposition of copper in the ventricular nucleus of the brain and kidney failure. In some instances, exposure to copper has resulted in jaundice and enlarged liver.

2.1.2 Health Effects Of Lead

Lead has a significant role in many industries because it is ductile and easily shaped. It has been used in many sectors and products: batteries, petrol additives, chemical compounds, pigments, and cables (National Mining Association, NMA, 2009).

Overdoses of lead and long-term exposure can tend to severe impacts especially on infants. High concentrations of lead may cause problems in the synthesis of hemoglobin, effects on the kidney, gastrointestinal tract, joints and reproductive system, and acute or chronic damage to the nervous system. According to the Environmental Protection Agency (EPA) the long-term exposure of lead can be severe and tends to decreased growth, hyperactivity, impaired hearing, and brain damage. Recent studies have stated that lead may have an impact on mental and psychological developments in children; for instance, children may lose up to 2 Intelligence Quotient (IQ) points if the blood lead level rises from 10 to 20 μg/dl. Lead mainly can be found in foods from the deposition of dust and rain containing lead on crops and soil. It can also accumulate in the human body from point source emissions. For example, lead can exist in drinking water from old lead piping and from illegal discharging of industrial waste water of high concentrations into surface fresh water.

Stricter regulations with regard to the metal discharges are being enforced particularly in industrialized countries. Toxicology of heavy metals confirms their dangerous impacts. Due to their high toxicity, industrial wastewaters containing heavy metals are strictly regulated and must be treated before being discharged in to the environment.

Table 2.1United States Environmental Protection Agency for electroplating Effluent standards (Frency Mathew, 2008)

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2.2 conventional methods

Several technologies have been used to treat metal containing aqueous solution for the last few decades (Wang S et al, 2005). Commonly used methods for removing metal ions from aqueous streams include chemical precipitation, lime coagulation, ion exchange, reverse osmosis and solvent extraction. However, these methods proved either inefficient or expensive in case of low concentration (1-100 mg/l) of heavy metals prevailing in the environment and generate huge amount of sludge which are difficult to dispose. Major drawbacks of the conventional processes can be summarized as follows (Suhag A et al, 2011):

Reverse Osmosis: It is a process in which heavy metals are separated by a semi-permeable membrane at a pressure greater than osmotic pressure caused by the dissolved solids in wastewater. The disadvantage of this method is that it is expensive.

Electro dialysis: In this process, the ionic components (heavy metals) are separated through the use of semipermeable ion selective membranes. Application of an electrical potential between the two electrodes causes a migration of cations and anions towards respective electrodes. Because of the alternate spacing of cation and anion permeable membranes, cells of concentrated and dilute salts are formed. The disadvantage is the formation of metal hydroxides, which clog the membrane.

Ultra-filtration: They are pressure driven membrane operations that use porous membranes for the removal of heavy metals. The main disadvantage of this process is the generation of sludge. Ion-exchange: In this process, metal ions from dilute solutions are exchanged with ions held by electrostatic forces on the exchange resin. The disadvantages include: high cost and partial removal of certain ions.

Chemical Precipitation: Precipitation of metals is achieved by the addition of coagulants such as alum, lime, iron salts and other organic polymers. The large amount of sludge containing toxic compounds produced during the process is the main disadvantage.

the demerits in the above-mentioned techniques like incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require careful disposal has led to look for cost-effective treatment method that is capable of removing heavy metals from aqueous effluents.

2.3 Bio sorption of heavy metals

Biosorption is defined as the capability of biological materials to accumulate heavy metals from wastewater through metabolically inactive or dead biomass. It is a process with some unique characteristics. It can effectively sequester dissolved metals from very dilute complex solutions with high efficiency. This makes biosorption a promising alternative for the complex waste-waters. Algae, bacteria, fungi and yeasts have proved to be potential metal bio sorbents. The major pros of bio sorption over conventional treatment methods include low cost, high efficiency, minimization of chemical or biological sludge, No additional nutrient requirement, and regeneration of bio sorbent and possibility of metal recovery. It has emerged as promising technique for metal removal. The processes can occur at an interface between any two phases, such as, liquid-liquid, gas-liquid, or liquid-solid interfaces (Barakat M. A, 2011).

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Figure 2.1 schematic diagram of bio sorption process of metal ions

2.3.1 Merits Of Biosorption

Bio sorption is growth-independent process.i.e. e, non-living biomass is not subjected to toxicity limitation of cells. Costly nutrients are not required for the growth of cells in feed solutions Thus, there is no be problem with the disposal of surplus nutrients or metabolic products. The process is not governed by the physiological constraint of living microbial cells. Since non-living biomass behave as an ion exchanger; the process is very rapid and takes place between few minutes to few hours. Metal loading on biomass is often very high, leading to very efficient metal uptake. Because cells are non-living, processing conditions are not restricted to those conducive for the growth of cells. In other words, a wider range of operating conditions such as pH, temperature and metal concentration is possible. Metal can be desorbed readily and then recovered (Abida Z et al, 2015).

2.3.2 Biosorbent Materials

I. Microorganisms

Microbial biosorbents are basically small particles, with low density, poor mechanical strength and little rigidity. Even though they have merits, such as high biosorption capacity, rapid steady state attainment, less process cost and good particle mass transfer, they often suffer several drawbacks. Biosorbents for the removal of metals mainly come under the following categories: bacteria, fungi, algae, industrial wastes, agricultural wastes and other polysaccharide materials. In general, all types of biomaterials have shown good biosorption capacities towards all types of metal ions. These Chromium (VI), Nickel (II), Copper (II) and Cobalt (II) heavy metal ions are approach with Bacillus subtilis, Pseudomonas aeruginosa and Enterobacter cloacae. It is a novel genus and species to the biosorption process for particular metals, this is very inaccessible theme to get definite references. Potent metal biosorbents under the class of bacteria include genre of Bacillus (Nakajima and Tsuruta, 2004), Pseudomonas (Chang et al, 1997; Uslu and Tanyol 2006) and Streptomyces (Selatnia et al, 2004) etc. Important fungal biosorbents include Aspergillus (Kapoor and Viraraghavan 1997; Jianlong et al 2001; Binupriya et al 2006), Rhizopus (Bai and Abraham, 2002; Park et al, 2005) and Penicillium (Tan and Cheng, 2003), etc. Since these micro-organisms are used widely in different food/pharmaceutical industries, they are generated as waste, which can be attained free or at low cost from these industries.

II. FUNGI

During past few decades, biosorption has been studied extensively using various biomasses such as white-rot fungus (Yetis et al, 2000; Bayramoglu et al, 2002), Saccharomyces cerevisiae (Goksungur et al, 2005; Ohnuki et al, 2005). Fungi and yeasts are easy to grow, produce high yields of biomass and can be manipulated genetically and morphologically. The fungal organisms are widely used in a variety of large-scale industrial fermentation processes. For example, strains of Aspergillus are used in the production of ferrichrome, kojic acid, gallic acid, itaconic acid, citric acid and enzymes like amylases, glucose isomerase, pectinase, lipases and glucanases; while S.cerevisiae is used in the food and beverage industries. The biomass can be cheaply and easily procured in rather substantial quantities, also as a byproduct from the established industrial fermentation processes, for the biosorption of heavy metals and radio nuclides, which made the fungi of primary interest as a raw material serving as a basis for formulating suitable biosorbents. The use of biomass as an adsorbent for heavy-metal pollution control can generate revenue for industries presently wasting the biomass and at same time ease the burden of disposal costs associated with the waste biomass produced.

III. WASTE MATERIALS

Agricultural biomass mainly consists of lignin, cellulose, hemicellulose and some proteins which make them effective biosorbent for heavy metal cations. Various investigated biomasses for their possible application in wastewater treatment for heavy metal removal include peapod, cotton and mustard seed cakes (Iqbal et al, 2002), sawdust (Garg et al, 2004), Ocimum basilicum seeds (Melo and Dsouza et al, 2004), waste tea (Mahvi et al, 2005), Cicer arientinum husk (Ahalya et al, 2005), sugarcane bagasse, maize corncob and Jatropha oil cake (Garg et al, 2007).

The utilization of agricultural waste materials is increasingly becoming a vital concern because these wastes represent unused resources and, in many cases, present serious disposal problems. Numerous waste biomass sources are available in different parts of the world, on which some experimental adsorption properties have been reported e.g. rice husk (Kumar and Bandyopadhyay, 2006; Wong et al, 2003; Zulkali et al, 2006).

IV. ALGAE

Higher uptake capacity has been found in brown algae than red and green algae (Brinza et al, 2007). The reason seems to be that they offer better sorption than red or green algae (Romera et al, 2006). Researchers have employed mainly brown algae (treated in different ways to improve their sorption capacity (Romera et al, 2006).The micro algae include Chlamydomonas reinhardtii, Chlorella salina, Chlorella sorokiniana, Chlorella vulgaris, Chlorella miniata, Cyclotella cryptica, Lingpa taylorii, Phaeodactylum tricornutum, Porphyridium purpureum, Spirogyra sp., Spirulina platensis, Stich coccus bacillaris and Stigeoclonium tenue. These algae were reported to be able to adsorb one or more heavy metal ions, including Co, Cu, Mn, Ni, Zn, Cd, with good metal uptake capacity (Brinza et al, 2007). Chojnacka et al (2005) reported the biosorption performance of Cr3+, Cd2+ and Cu2+ ions by blue–green algae Spirulina.

Table 2.2 Comparison of bio sorption maximum capacity of Zn (II) on different bio sorbent

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2.3.3 Gaps in The Literature

Literature review indicates that different forms of biomass can be used for the removal of heavy metal pollutants in the industrial effluents. Investigation has been carried out to evaluate the performance of easily available adsorbent for removal of metals from aqueous solution. Undoubtedly, such low-cost adsorbents offer a lot of promising benefits for their commercial use in the future. However, only few researches were done on biosortpion studies using lemna minor as bio adsorbent. Thermodynamic effects on biosorption process have been insuffiently discussed. Interaction effects that could occur among process variables were neglected. The desorption of the used adsorbent which is essential for possible metal recovery was not given emphasis (Handojo et al, 2016, Tasrina et al, 2015, Balarak et al, 2015); therefore, the current research was conducted with the main objective of filling the identified gaps. The search for new technologies involving the removal of toxic metals from wastewaters has directed attention to natural biomass. They have also proven useful in removing organic wastes from waste water. Some examples are cattails, calla lilies, arrowhead, ginger lilies, pickerelweed, water hyacinths, Lemna minor (Duckweed), water lettuce, water spinach, aquatic mosses and liverworts. The use of Lemna minor as a bio adsorbent has a lot of merits over other bio adsorbents. It is fast to grow (twenty times faster than corn), renewable, and tolerate wide range of pH (Suhag A et al, 2011).

2.4 Characteristics of Lemna Minor

Lemna minor is a stem less, small, and free floating aquatic plant that belong to Lemnaceae family (Cheng et al, 2002). The Lemnaceae family consists of four genera (Lemna, Spirodela, Wolffia and Wolffiella) and 40 species have been identified. They are green and have a small size (1-3mm), with short but dense roots (1-3cm) (Sooknah R et al, 2004). Leman minor (duckweed) generally grows on the surface of still or slow-moving water. Duckweed fronds grow in colonies that, in particular growing conditions, form a dense and uniform surface mat (Hasar et al, 2000). They are found world-wide on the surface of nutrient rich fresh and brackish waters (Zimmo, 2003) but the greatest diversity appears in subtropical and tropical areas. Their habitat comprises still or slowly moving fresh or polluted waters of only a few mm to 3m depth. In particular nutrient-rich and sheltered small ponds, ditches and swamps, e.g. down-stream from sewage works, often contain duckweed.

Abbildung in dieser Leseprobe nicht enthalten Abbildung in dieser Leseprobe nicht enthalten

Figure 2.2 lemna minor leaves A (fresh lemna minor), B, acid treated, ground and dried)

2.4.1 Previous study of Biosorption on to lemna minor

Tasrina et al (2015) investigated the removal of Arsenic (III) from ground water by adsorption onto lemna minor. The study showed that maximum arsenic (III) removal was obtained under the following conditions: initial As (III) concentration, 100 μg/L; Duck weed amount, 3 g; average particle size, 0.595 mm, and pH, 5.5, respectively.

Balalark et.al (2015) studied the effect pH, initial dye concentration, adsorbent dose and contact time on adsorption capacity of lemna minor (duck weed) biomass. It was found that lemna minor was able to remove 98% acid red 88 dye from aqueous solution. The resulting adsorbent was characterized its ultimate analysis, morphological structure and its surface area. The ultimate analysis of a dry duckweed was as the following (%): C − 39.11; H − 6.13; O − 37.74; N − 5.52; S − 0.67; balance − mineral matter. Scanning electron microscopy (SEM) images of modified lemna minor showed the adsorbent had heterogeneous surface structure with deep pores. The specific surface area of modified Lemna minor was determined in size of 30 m2/gr.

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Figure 2.3 scanning electron microscopy image of modified Lemna minor before and after used as an adsorbent (source, Balarak et al.2015)

2.5 Adsorption Mechanism

The cell wall is the first component that comes into contact with metal ions, where the solutes can be deposited on the surface or within the cell wall structure. The solute uptake by live/dead cells is extracellular, the chemical functional groups of the cell wall play vital roles in bio sorption. Due to the nature of the cellular components, several functional groups are present on the bacterial cell wall, including carboxyl, phosphonate, amine and hydroxyl groups (Van der Wal et al, 1997).

Biomass possess an abundance of functional groups that can passively adsorb metal ions. The term adsorption can be used as a general term and includes several passive, i.e. non-metabolic, mechanisms such as: complexation; chelation; co-ordination; ion exchange; precipitation; reduction. For adsorption to occur, there must be forces that attract the adsorbate to the solid surface in a solution. This mechanism or forces which attract the adsorbate to the solution of the solid interface can either be physical or chemical (Mckay, G. 1996).

Physical Adsorption

Physical adsorption (physisorption) is a reversible method in which there is the attraction of molecules by mechanical forces when the molecules come in contact with the adsorbent. The reversible process depends basically on the force of attraction between the adsorbate and adsorbent. This type of adsorption is multilayer which means that each molecule layer forms on the top of previous with the number layers being proportional to the contaminate concentration (Chiron N et al, 2003).

Chemical Adsorption

In comparison to physical adsorption, chemical adsorption is an irreversible process which is caused as a result of the reaction taking place between molecules of the adsorbed substance and the adsorbate. It involves the formation of covalent or ionic bonds, consumes high energy and it can occur over wide range of temperature. “Due to its irreversibility, monolayer is expected to form chemisorption while multilayer is encountered in physisorption” (Mckay G, 1996).

Complexation

As noted above complex formation of metal ions with organic molecules involves ligand centres in the organic species i.e. the presence of an atom or atoms having lone pair electrons to donate. Complexation may be electrostatic or covalent and the simplest case is complexation by a mono-dentate ligand such as RNH2. To approach and elucidate biosorption mechanisms, a significant part of the recent advances in biosorption are based on the classification of elements according to the hard-soft acid-base classification (Pearson’s classification). “Hard acids”, metals such as Na, K, Ca, Mg, often essential nutrients for microbial growth, bind preferentially to oxygen containing “hard bases”, ligands such as OH- , HPO4 2-, CO3 2-, R-COO- , and =C=O. Soft acids, metals such as the precious metals Ag, Au, Pt, Pd are bound covalently to the cell wall by “soft bases”, ligands containing nitrogen or sulfur. As noted earlier several mechanisms might be involved in the immobilization of metals and it is now evident and confirmed by several researchers, that the biosorption of precious metals is a two-step mechanism comprising first covalent bonding and then in-situ reduction (Beveridge et al, 1976). Borderline ions on the “hard/soft” classification such as the transition elements: Mn2+, Zn2+, Cd2+, Cu2+, Pb2+ exhibit a significant degree of covalent bonding with nitrogen or sulfur containing “soft” ligands (Davis et al, 2003).

Chelation

Organic molecules containing more than one functional group with donor electron pairs can simultaneously donate these to a metal atom. This can result in the formation of a ring structure involving the metal atom a process termed ‘chelation’. In general, since a chelating agent may bond to a metal ion in more than one place simultaneously, chelated compounds are more stable than complexes involving mono dentate ligands. Stability tends to increase with the number of chelating sites available on the ligand. Thus chelation of metals by donor ligands of biopolymers leads to the formation of stable species. Co-ordination Metal atoms have preferences for specific donor atoms (“hard/hard” / “soft/soft”) and the stereo chemical arrangements that play an important role in the binding with the available ligands on the microbial cell. Limited information of surface complexation models, based on the theory of surface co-ordination chemistry, is available to describe metal bio sorption

Anion Exchange

Anion exchange on biopolymers can take place on a variety of organic-nitrogen-based groupings. In proteins, amino (lysyl side chain and N-terminal) imidazole (histidyl) and guanidine (arginyl) groupings are common centres of positive charge. Centres of positive charge in nucleic acids will occur with protonation of amino groups on purine or pyrimidine rings or with protonation of heterocyclic nitrogen atoms. Polysaccharides as a group are acidic or neutral macromolecules with basic functional groups being rare and arising as un-acetylated amino sugars. Chitin is the notable example where a proportion of glucosamine residues are reportedly un-acetylated and which provides on deacetylation chitosan with a high proportion of protonisable positive charge centres. Many examples have established the existence of an ion exchange mechanism in metal ion removal by biosorption (Vasudevan, 2002). However, it has been suggested by many researchers that ion exchange is neither the sole nor the main mechanism for metal biosorption (Brady, 1995).

P recipitation

Metal precipitation is also involved in biosorption. The precipitates may be formed and remain in contact with or inside the microbial cells or may be independent of the solid phase of the microbial cell. In the later case, the presence of the solid phase-microbial cell or biofilm also plays a favorable role in the phenomenon of precipitation. The term precipitation in most cases refers to the formation of insoluble inorganic metal precipitates (Remoundaki, 2003). However, in the case of metal biosorption by microbial cells, organic metal precipitates may also be formed. This may be more easily understood when metals are bound to Extracellular Polymeric Substances (EPS) excreted by some prokaryotic (bacteria, archaea) and eukaryotic (algae, fungi) microorganisms. Purified products from isolated cells such as glucan, mannan, and chitin accumulate greater quantities of cations than the intact cells, proving that biomolecules can form metal precipitates.

Reduction

The removal of toxic hexavalent chromium from aqueous solution by biosorption by different biomass types has been extensively reported. This removal is often associated with the simultaneous reduction of Cr (VI) to Cr (III), thus inactivated fungal biomass e.g. Aspergillus niger, Rhizopus oryzae, Saccharomyces cerevisiae and Penicillium chrysogenum remove Cr (VI) from aqueous solutions by reduction to Cr (III) when contacted with the biomass (Donghee, 2005). Also soft metals like gold and palladium are first bound on sites on and within the cell wall and these sites act as nucleation points for the reduction of metals and growth of crystals and elemental gold and palladium have been obtained. The biosorption mechanism is a two-step process: initiation of the uptake at discrete points by chemical bonding, then reduction of the metal ions (Lin et al, 2005).

2.6 Factors Affecting Biosorption

The most important factors that should be taken into account when considering biosorption are: the type and nature of the dosage, initial solute concentration, adsorbent dosage and physicochemical factors like temperature, pH, and ionic strength (Chimie, 2014).

Apart from the physicochemical factors such as pH, the presence of other anions and cations, metal speciation, pollutant solubility and form, etc. may also have an influence (Omran, 2015).

pH

The PH of the metal ion solution is an important parameter for adsorption of metal ions because it affects the solubility of the metal ions, concentration of the counter ions on the functional groups of the adsorbent and the degree of ionization of the adsorbate (Ii et al, 2011).Therefore, pH of solution influences the nature of biomass binding sites and metal solubility; Metal adsorption has frequently been shown to be strongly pH dependent in almost all systems examined, including bacteria, cyanobacteria, algae, and fungi (Holan & Volesky, 2007).

It has been generally reported that in highly acidic medium (pH≈2) the removal of metal ions is almost negligible and it increases by increasing the solution pH up to a certain limit (Abdelghani & Elchaghaby, 2014).

The decrease of adsorption levels by lowering pH can be due to competition between protons and metal ions for capturing same sites (Igwe, 2006). On the other hand, too high pH value can cause precipitation of metal complexes, so it should be avoided during experiments. For different adsorption system of metal ions, the optimal pH is different (Wang & Chen, 2006).

Temperature

The effect of temperature is fairly common and increasing the mobility of the metal cation (Malkoc & Nuhoglu, 2005). Temperature influence has more effect in a situation where by metal uptake increases within a temperature range of about 20-30 0C, but decreases with an increase of temperature above a critical value. (Sulaiman, 2015) Increase in temperature probably weakens the bond formed between the metal ions and the adsorption sites on the adsorbent thereby resulting in an increase in the amount of metal ions adsorbed on the adsorbent. This implies that increase in temperature creates a wider surface area for adsorption at the adsorbent (Okafor et al, 2012). At high temperature, the thickness of the boundary layer is expected to decrease due to the increased tendency of the metal ion to escape from the surface of the adsorbent to the solution Phase hence there was bound to be weak adsorption interactions between the adsorbent and the adsorbate (Ojedokun & Solomon, 2016).

Contact time

Adsorption is also affected by contact time between biomass and the solution containing metals. Adsorption proceeds fast and most metals are adsorbed at the very beginning of the process (Zabochnicka & Krzywonos, 2014). In adsorption systems, contact time plays a vital role irrespective of other experimental parameters affecting the adsorption kinetics. The determination of the optimum contact time needed to achieve the highest removal of metal ions is very important in batch adsorption experiments. The study on removal of iron, nickel and zinc was analyzed by ( Elmaghrabi, 2014) that the time-dependent behavior of metal ions was measured by varying the equilibrium time in the range of 30–240 min. Results showed that the equilibrium is reached quickly (~30 min), indicating that, the adsorption sites are well exposed.

Initial solute concentration

It is generally agreed that the adsorption capacity increases as the initial metal ion concentration in the solution increases, whereas the metal removal percentage (also called removal efficiency) decreases by increasing the metal ion initial concentration (Islam M et al.2015). As a rule, increasing the initial metal concentration results in an increase in the adsorption capacity because it provides a driving force to overcome mass transfer resistance between the adsorbent and adsorption medium (Pahlavanzadeh et al, 2010).

Adsorbent dosage

The amount of biomass in the solution also affects the specific metal uptake. In principle, with more adsorbent present, the available adsorption sites or functional groups also increase (Mosbah & Sahmoune, 2013). At low biomass dosage, the number of ions adsorbed per unit adsorbent weight is high. Adsorption capacity is reduced when the biomass dosage increases as a result of lower adsorbate to binding site ratio where the ions are distributed onto larger amount of biomass binding sites (Ojedokun & Solomon, 2016).

2.7 Adsorption Equilibrium Model

Adsorption equilibrium is the basic requirement for designing adsorption system (Chimie, 2014). The adsorption isotherms describe the relationship between the mass of the adsorbed component per adsorbent mass and the concentration of this component in the solution ((Islam M et al, 2015). Modeling equilibrium sorption is important for industrial applications of adsorption; it yields data that facilitates designing and optimizing the process. During adsorption, a rapid equilibrium is established between adsorbed metal ions and adsorbent. The equilibrium metal uptake is calculated using the following equation:

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Where q is the amount of metal biosorbed by biomass (mg/g); Co is the initial concentration of metal (mg/L); Ce is the concentration of metal (mg/L) at equilibrium; V is the volume of the metal solution(L); and M is the mass of adsorbent (g) (Can and Jianlong, 2007).The sorption uptake can be expressed in different units depending on the purpose of the exercise: for example, milligrams of solute sorbed per gram of the(dry) biosorbent material (the basis for engineering process–mass balance calculations), or mmol/g (when the stoichiometry and/or mechanism are to be considered). A biosorption isotherm, the plot of uptake (q) versus the equilibrium solute concentration in the solution (Cf), is often used to evaluate the sorption performance.

2.7.1 Adsorption Isotherms

Adsorption process is usually studied through graphs known as adsorption isotherm. The equilibrium adsorption isotherm is important in the design of adsorption systems. Although several isotherm equations are available, but important isotherms including Langmuir, Freundlich and Temkin isotherms was studied. Langmuir and Freundlich sorption isotherm models consider sorption by free binding sites rather than ion exchange. Sorption isotherms are useful in quantitatively evaluating and predicting the process performance of the binding capacity and affinity for different metal concentrations and sorbent dosages (Bohli T et al, 2015). These models can be applied only at a constant pH. These models are mostly used for the modeling of adsorption equilibrium in the presence of one metal. Langmuir model is based on the simplest model of adsorption with the basic assumptions including 1) molecules are adsorbed at discrete active sites on the surface, 2) energetically uniform adsorbing surface, 3) each active site adsorbs one molecule only, 4) no interaction among the adsorbed molecules (Dada A et al, 2012). The Langmuir isotherm model can be represented by equation 2.2. Sorption isotherms are useful in quantitatively evaluating and predicting the process performance of the binding capacity and affinity for different metal concentrations and sorbent dosages (Isabel et al, 2014).

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The values KL and qm are computed from the slope and intercept of the Langmuir plot of 1/qe versus 1/Ce, respectively.

The Freundlich isotherm is one of the most popular isotherm which gives reasonable description of the adsorption of liquid molecules on solid surfaces. It is derived by assuming that the amount of substance adsorbed at equilibrium has a power law dependence on the concentration of the solute (Dada, A et al, 2012). Freundlich Isotherm model can be formulated by equation 2.3 Where qe = amount of heavy metal ion removed at equilibrium (g), Ce = concentration of the adsorbate at equilibrium (mg/l), qm = initial amount of heavy metal (g), X/m = adsorption per gram of adsorbent which is obtained be dividing the amount of adsorbate (x) by the weight of the adsorbent (m), KL and Kf are constants.

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Temkin isotherm contains a factor that explicitly takes into the account adsorbing species-adsorbent interactions. This isotherm assumes that (i) the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent-adsorbate interactions, and that (ii) the adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy (Mall et al, 2005). A plot of qe versus ln Ce enables the determination of the isotherm constants B1 and KT from the slope and the intercept, respectively. KT is the equilibrium binding constant (l/mol) corresponding to the maximum binding energy and constant B1 is related to the heat of adsorption. Temkin isotherm is given in equation 2.4.

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This can be linearized as:

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Where

2.7.2Adsorption Kinetics Models

The prediction of adsorption rate gives important information for designing batch adsorption systems (King P et al, 2007). The kinetics of adsorption was studied by using pseudo first order, pseudo second order and intraparticle diffusion models as the most important kinetic models (Balarak et al, 2015). Adsorption kinetics describes the rate of solute uptake, which is also responsible for the residence time needed for an adsorption study. Therefore, it is an important characteristic in defining how fast the sorption processes is carried out.

Pseudo first-order kinetics model

The Pseudo first-order rate kinetics model based on adsorption capacity of adsorbent is generally expressed as: (Zazouli et al., 2014):

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The plot of log (qe – qt) versus t should give a linear relationship from which K1 and qe can be determined from the slope and intercept of the plot, respectively.

Pseudo second-order kinetics model

The Pseudo-second order kinetics model is tested in the same way by rewriting the equation in its linear form and plotting the appropriate variables. The equation that describes the pseudo-second order model is given in the following linear form (Zazouli et al, 2014)

The constants, qe and K2, are obtained from the slope and intercept of t/qt versus t linear plot, respectively.

Intraparticle diffusion model

The nature of the rate-limiting step in adsorption system could be assessed from the properties of the adsorbate and adsorbent as the metals were probably transported from its aqueous solution to the adsorbent by intra-particle diffusion (Santhy et al, 2006). Therefore, to investigate whether there was a possibility of adsorption of the adsorbate to diffuse into the interior pores of the adsorbent or not, intra-particle diffusion was explored by using Weber and Morris equation. (Weber and Morris, 1963).

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Where, q t is the amount of metal adsorbed at time‘t’ (mg g-1), Ki is intra-particle diffusion rate constant (mg g-1 min-1/2) and C is intercept. The value of Ki and C were obtained from slope and intercept of the linear plot of qt versus (t0.5) at different initial metal concentrations and room temperature 250c. When the metal ion solution is mixed with the adsorbent, transport of the metal ions from the solution through the interface between the solution and the adsorbent occurs into pores in the particles. There are four main stages in the process of adsorption by porous adsorbents (i) solute transfer from the bulk solution to the boundary film that surrounds the adsorbent’s surface, (ii) solute transport from the boundary film to the adsorbent’s surface, (iii) solute transfer from the adsorbent’s surface to active intra particular sites, and (iv) interactions between the solute molecules and the available adsorption sites on the internal surfaces of the adsorbent. One or more of these four steps controls the rate at which solute is adsorbed.

3. MATERIALS AND METHODS

3.1 Equipment And Chemicals

The main chemicals used for the experimental works were lemna minor adsorbent, 0.1M sodium hydroxide, 0.1M hydrochloric acid, lead nitrate (Pb(NO3)2), Copper Sulphate (CuSO4. 5H2O), Sodium chloride (NaCl) and distilled water. All Chemicals were obtained from chemical stores in Addis Ababa. The bio sorption process has different stages and hence used a number of instruments and equipment. Lemna minor leaves after dried was ground using a grinder. The samples of the bio adsorbent were weighed using analytical balance. Samples were placed in a conical flask for measuring the volume and were titrated using pipette. Both adsorbate and adsorbent were well mixed by using incubator shaker. Samples were heated in an oven and were kept in glass beakers. The functional groups present in the bio adsorbent were analyzed by FTIR spectra. The specific surface area of the adsorbent was measured by using sears method. The concentration of the metal content in the solution was measured using UV VIS spectrophotometer through calibration curve. Chemicals such as salts of copper and lead were used to make an aqueous solution that is ready for adsorption. 0.1M Hydrochloric acid was used to treat the bio adsorbent and adjust the pH of the solution. 0.1M Sodium hydroxide (NaOH) was used for adjusting the pH; Sodium chloride was used for making a solution to be used in surface area determination by sears method. Tong and stop watch were needed for handling hot beakers and time keeping respectively. Masks, safety cloth and glove were used for safety.

3.2 Experimental Methods

3.2.1 Preparation Of Acid Treated Dried Powder Of Lemna Minor

Lemna minor was collected from around Hawasa called “Tikur haik”. They were washed with distilled water several times to remove dust and fines until the color of the wash water became transparent. After that, the washed duck weed was completely dried in order to reduce moisture content. Then, it was grounded with the grinding mill to form a powder. The ground duckweed was sieved through a fine mesh (820-850μm) (Davoud et al, 2016). Activation of the biomaterial was carried out by soaking the lemna minor powder in 0.1 M HCl solution for 24 hr under slow stirring. The solution pH was kept constant at 5.0 using 0.1 M of NaOH. The acid treated biomaterial was washed with distilled water to remove excess HCl. The activated samples were dried at 60°C for 24 hrs and stored at an airtight plastic bottle for later use (Zazouli et al, 2014).

3.3 Characterization Of Lemna Minor

3.3.1 Proximate Analysis Of Lemna Minor

Proximate analysis ash content, volatile matter, fixed carbon and moisture content of the duck weed was measured using the method of ASTM.

Determination of Moisture Content

The moisture contents of the duck weed samples was determined using ASTM, E-871 (2013) procedure. A powdered sample of duckweed with particle size in the range of 820-850 μm weighing 10 g was taken in a pan and placed inside a hot air oven at a temperature of 105±3°C. The sample was weighed at regular intervals and once the weight observed became constant, the moisture content was calculated using the formula:

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Determination of ash content

The ash content of the duck weed biomass was determined following the ASTM D 482-13 standard procedure. Biomass sample of 2g size from the oven dried sample was taken in crucible and placed in a furnace at 575±25°C for a period of 4 hr then it was cooled to room temperature in a desiccators and its weight was recorded. Then again it was placed in furnace and was dried to a constant weight. The percentage of ash in the sample was determined using the expression.

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Determination of volatile matter

1 g of dried sample was weighed into a crucible and incinerated at 850°C for 10 minute in furnace. This procedure is undertaken without contact with air in closed crucible. The crucible was then cooled in desiccators and reweighed (ASTM, 1989). The % volatile of the sample was calculated as follows:

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Determination of fixed carbon content

The fixed carbon (ASTM D3172 – 13) of samples was calculated by subtracting the sum of ash content (%) and volatile matter (%) from 100. The fixed carbon is the residue left after removing the volatile matter and the ash from the substance.

Fixed carbon (%) = 100 – % [ash content + volatile matter +moisture content] ..3.4

3.3.2. Fourier Transform Infrared Spectroscopy (FTIR) Analysis Of Lemna Minor

FTIR analysis was carried out in order to identify the functional groups that might be involved in the binding of heavy metal ions. The functional groups of raw, treated and metal loaded lemna minor were determined using FTIR Spectrometer (Spectrum 65 FT-IR, PerkinElmer) at wave number range of 4000- 400 cm-1. First, the dried adsorbent was mixed with KBr particles to make it suitable to infrared analysis. The mixture was then pressed to a small thickness, slightly below 1 mm, required for FTIR analysis. FTIR analysis was done at Addis Ababa University, College of Natural Science, and Chemistry Laboratory.

3.3.3 X ray Diffraction Spectroscopy Analysis Of Lemna Minor

X-ray diffraction spectroscopy (XRD) analyses were carried out at Addis Ababa university chemistry laboratory for both raw and acid treated Lemna minor adsorbent with Analytical X-ray, Philips Analytical. XRD analysis was made to identify the crystallographic structure of the lemna minor adsorbent. A dried sample of the adsorbent was ground using mortar and pestle and tested at 40kV and 40mA.

3.3.4 Point Of Zero Charge

A total of ten 200 ml of 0.1M NaCl solution conical flasks were prepared and their initial pH values were adjusted to 2, 3, 4, 5,6 7, 8, 9, 10 and 11 by adding 0.1M NaOH or 0.1M HCl solution by using a pH meter. 0.2 g of lemna minor adsorbent was added into each solution. The solution mixtures were allowed to equilibrate in an encubater shaker at 200 rpm and 250c for 1 hr. Then the solution was filtered and the filtrates final pH was measured. The results were plotted with ∆pH against pH final. The point at which ∆pH = 0 is known as pHPZC (Yadav O et al, 2012).

3.3.5 Specific Surface Area

The Lemna minor specific surface area was determined using the method described by Yadav et al, 2011. 1.0 g of adsorbent was mixed with 100 ml of distilled water and 20 g NaCl. The mixture was shaken for five minutes. Its final pH was adjusted to 4 with 0.1 M HCl. It was then titrated against 0.1M NaOH to raise the pH from 4 to 9 and the volume (ml) of 0.1M NaOH used was measured in replicate and the average value was taken for the surface area calculation by sears method. Specific surface area of adsorbent was calculated using the formula Sears method (Yadav O et al, 2012):

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