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Potential of dissolved organic matter (DOM) to extract As, Cd, Co, Cr, Cu, Ni, Pb and Zn from polluted soils: A review
- Borggaard, Ole K., Holm, Peter E., Strobel, Bjarne W.
- Geoderma 2019 v.343 pp. 235-246
- EDTA (chelating agent), aluminum oxide, arsenic, cadmium, cations, chromium, cobalt, composting, copper, dissolved organic matter, ecological footprint, economic analysis, environmental factors, food waste, fulvic acids, guidelines, heavy metals, human health, lead, ligands, microbial activity, moieties, nickel, pH, phytoaccumulation, pollutants, polluted soils, salicylic acids, soil pollution, soil remediation, soil structure, soil washing, solubility, sorbents, vanadium, zinc
- Pollution of soils with As, Cd, Co, Cr, Cu, Ni, Pb and Zn, collectively termed heavy metals (HMs), can threaten human health and ecosystem functioning. Therefore, polluted soils must be remediated. Soil washing of strongly polluted soils and phytoextraction on moderately polluted sites seem currently the most attractive remediation methods. Both methods depend on HM solubility (extractability), which can be increased by addition of ligands such as EDTA, NTA and other aminopolycarboxylic acids. As an alternative to these synthetic and environmentally questionable chemicals, the possibility of using naturally occurring dissolved organic matter (DOM) to extract the HMs from anthropogenic polluted soils is evaluated in this review based on mainly recently published laboratory studies. DOM isolated or extracted from soils consists of fulvic acid (FA) with up to 10% low-molecular-weight-organic-acids (LMWOAs), e.g. citric, oxalic and salicylic acids. In addition to soil DOM, results with other soluble humic substances and organic waste materials have been included, e.g. food wastes that after composting give useful LMWOAs.Although generally less efficient than EDTA and NTA, the review clearly shows potential of DOM as HM extractant. Through its carboxylate and/or phenolate functional groups, DOM can form soluble complexes with Cd2+, Co2+, Cu2+, Ni2+, Pb2+ and Zn2+ (and other di- and trivalent cations). Extraction of anionic As(III), As(V) and Cr(VI) sorbed by soil solids, mainly amorphous Al and Fe oxides, is accomplished by DOM dissolution of the sorbents (oxides). In addition to the soil HM extraction potential, use of DOM will probably extract organic pollutants, stimulate microbial activity and improve soil structure. On the other hand, the efficiency of DOM to extract HMs (and other pollutants) seems to depend of numerous variables/factors including the specific HM, DOM and polluted soil as well as environmental (external) conditions such as pH, solution:soil ratio, extraction time and metal loading of the extractant. Furthermore, several different studies have shown conflicting (inconclusive) results, e.g. of pH and ageing effects on HM extractability. Therefore, much is still to be learnt about different DOM-HM-soil systems and their dependency of external factors. To create operational and safe guidelines for using DOM as HM extractant in relation to remediation of polluted field soils by phytoextraction or soil washing, considerably more precise knowledge is needed on influence of composition of different DOM samples and of characteristics of the polluted soils for extraction of the different HMs under various, clearly specified environmental conditions. If future investigations confirm the suitability of DOM in soil remediation, the next challenges will be upscaling to field conditions as well as to ensure availability of enough good quality DOM and to complete state-of-art economic analysis of using DOM in operational phytorextraction and soil washing.