The green chemistry method was used to extract humic acid from sediments obtained from Eniong River, Nigeria). The percentage yield of 0.2, 0.41 and 0.63 were obtained for 0.1, 0.5 and 1.0M NaOH respectively. The prepared humic acid (HA) was characterized using Ultraviolet-visible spectroscopy, Fourier transformed infrared spectroscopy (FTIR) and as well an investigating its surface properties. The spectrum confirmed the presence of surface functional group including -OH, -CO, -COOH. The UV spectrum was featureless typical of humic acids. The surface properties investigation showed that the extracted humic acid (EHA) was a mesoporous adsorbent with pore size between 2-50nm.
Background of Study
The first documented attempt to isolate humic substances from soil appears to have been made by Achard in (1786) . He extracted peat with alkali and obtained a dark, amorphous precipitate upon acidification. “Humus’’ (Latin equivalent of soil), a term which is usually credited to de Saussure was used to describe the dark coloured organic material in soil (de Saussure, 1804).
Dobereiner designated the dark coloured component of soil organic matter as “humussaure” or “humus acid”. This term came to be used synonymously with “humic acid” although Waksman concluded that “humus acid” was, for the most part, an inclusive term for all “humic acids” whereas the latter was applied to the precipitate obtained from the alkali extract by acidification (Dobereiner, 1822; Kononova, 1966).
Extended research on the chemical properties of humic substances was conducted by Swedish investigator (Berzelius, 1839) and the concept that isolated components of humus into chemically individual compounds (humic acid, crenic acid, apocrenic acid) was challenged by the Russian investigator German (Kononova, 1966). A detailed study of the nature and structure of humic substances was carried out by Shmook (Shmook, 1930). The view that lignin was the precursor of humic acids was further advanced by Fuch (1930) as well as Hobson and Page (1932).
Despite Waksman’s recommendation, terms such as humic acid, humin, fulvic acid, and others survived and will undoubtedly continue to be used in the future. The majority of studies conducted on humus involve preliminary separations on the basis of solubility characteristics, and abandonment of the terms would cause even greater confusion than their continued use (Stevenson, 1994).
Humic substances (HS) are major components of the natural organic matter (NOM) in soil and water as well as in geological organic deposits such as lake, sediments, peats, brown coals and shales (Eglinton and Murphy, 1969; Malcolm, 1990; Kraska, Szyper and Roacrowicz, 1994). A particularly interesting concept of the nature of humic substances is that the various fractions obtained on the basis of solubility characteristics are part of a heterogeneous mixture of molecules, which, in any given soil, range in molecular weight from as low as several hundred to perhaps over 300,000 and that exhibit a continuum of any given chemical property. The low-molecular-weight fulvic acids have higher oxygen content but lower carbon content than the high-molecular-weight humic acids. (Flaig, 1988; Hedges, 1988). Some postulated relationships are depicted in Figure 1 below.
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Figure 1 Classification and chemical properties of HS, from Stevenson (1982).
The precise chemical nature and properties of humic substances depend to some extent on the environment in which they are found e.g. soil, peat bog, surface water, ground water, lakes, streams, oceans etc. Various humification mechanisms have been postulated. Humification is a dynamic process involving competing formation, degradation reactions and local concentrations of humic substances usually vary seasonally (Biedebeck, 1978).
Soil humics are derived from terrestrial plants. The presently accepted view of one major mechanism for their production is that micro-organism acting on decomposing plant components synthesizes polyphenols (quinones). These compounds further polymerize, alone and/or in the presence of amino compounds to form the yellow-brown coloured humic compounds (Camire, 1991).
Another mechanism suggests that lignin is the main pre-cursor. Lignin is a largely aromatic material present in terrestrial vascular plants. The polyphenols are believed to be released by the degradation of lignin in the initial step in the process.
The third postulate a mechanism that gives rise to different humic structures which involve condensation of amino acids and related substances with the reducing sugars that are present in terrestrial plants (Jardine, Weber and McCarthy, 1989).
Some water-borne humic substances are of course simply leachate from the land so they have their origins in soil humification processes and have similar properties. However, other aquatic humics are undoubtedly derived from decomposition of aquatic plants which generally do not contain lignin so the lignin mechanism cannot be applied. Aquatic humics are known to be generally more aliphatic than terrestrial humics (Kogel-Knabner, 1993).
A different mechanism, postulated for marine humics, is that they are generated from material released by phytoplankton, small free floating micro-organism (Mortland, 1986). Overall, it is probable that different mechanisms predominate in different environments. Example, the lignin pathway in a peat bog, sugar amino acid route in soils in harsh continental situations and phytoplankton route in lakes, seas and oceans.
In summary, Humic acidsare the fraction of humic substances that is not soluble in water under acidic conditions (pH < 2) but is soluble at higher pH values. They can be extracted from soil by various reagents and which is insoluble in dilute acid. Humic acids are the major extractable component of soil humic substances. They are dark brown to black in colour. It comprises of Humin which is insoluble in acid or base and Fulvic acid which is soluble in base and acid Humic acid (HA) is a principal component of HS, which are the major organic constituents of soil (humus), peats, coal, many upland streams, dystrophic lakes, and ocean water. It is produced by biodegradation of dead organic matter. It is not a single acid; rather it is complex mixture of many different acids containing carboxyl and phenolate groups so that the mixture behaves functionally as a dibasic acid or, occasionally, as a tribasic acid. HAs can form complexes with ions that are commonly found in the environment creating humic colloids (Stevenson 1994; Waksman, 1936).
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Figure 2 Structure of Humic Acid (Stevenson, 1982).
The effective method to remove dye is to use an adsorbent to adsorb the dye molecules, which removes the colour from water. Recently, interaction between humic acid and metal ions has been given prominent attention (Qadeer, 2007). However, there is no report on the adsorption of dyes by humic acid. Although a number of adsorbents have been reported to remove methylene blue from wastewater, high cost, high-energy requirement and production of toxic sludge or waste that require further disposal are some of the setbacks of their use (Bellock, 1971). There is great need to develop effective and low cost adsorbent for the removal of dyes from wastewater.
2.1 Description of Study Area
The benthic sediment was collected fromEniong River, located at Okopedi Itu in the south-eastern coast of the Niger Delta-Nigeria. The humic freshwater ecosystem of Eniong River is a tributary of the lower course of the Cross River named after Cross River State which occupies 20,156 square kilometres. It shares boundaries with Benue to the north, Enugu and Abia States to the west, to the east by Cameroun Republic and to the south by Akwa-Ibom and the Atlantic ocean.
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Figure 3.1: The mid-stream of Cross River showing the location of Eniong
Creek (Cross River State Geographic Information Agency) (CRGIA, 2012)
2.2 Collection of Samples
Sediments were obtained using hand trowel. The benthic sediment was obtained in the month of July 2013. Precautions were taken to avoid contamination. The samples were taken to the laboratory for drying as soon as possible in order to minimize changes in the concentration of extractable nutrients and some organic constituents. The samples were air-dried by spreading out thinly in a highly ventilated room with occasional stirring to expose new surface and facilitating drying process. The dried samples were ground into powdery form using corona grinder. The powdery samples were stored (in a sealed container and kept in the dark to prevent photo-oxidation of the organic matter in the sediment) for both physical and chemical analysis (US EPA, 2001).
2.3 Determination of Physical Properties and Nutrient Availability in River Sediment
2.3.1 Particle Size Analysis by Hydrometer Method
Fifty grams (50 g) of air-dried sediment was weighed into a mechanical stirrer cup. Distilled water was added up to one and half inches to the top. This was followed by 20 ml of the dispersing agent. The bottle was inserted into the cup with the stirrer blade and the content stirred for 10 mins. The cup was removed and the content poured into a measuring cylinder. The suspension in the cylinder was made up to the 1000 ml mark with distilled water.
The temperature of the solution was recorded and the content of the cylinder shaken vigorously. The hydrometer was placed into the suspension after 30 seconds. The hydrometer reading was recorded after one minute. The hydrometer was washed and another reading taken after 2 hours alongside with the temperature (Andrés N. B., Ana V. S., Leonardo P., Deborah T., Daniel B., Raquel M and Adriana G., 2014).