Girish Choppala

Role of organic amendments on enhanced bioremediation of heavy metal(loid) contaminated soils.

As land application becomes one of the important waste utilization and disposal practices, soil is increasingly being seen as a major source of metal(loid)s reaching food chain, mainly through plant uptake and animal transfer. With greater public awareness of the implications of contaminated soils on human and animal health there has been increasing interest in developing technologies to remediate contaminated sites. Bioremediation is a natural process which relies on soil microorganisms and higher plants to alter metal(loid) bioavailability and can be enhanced by addition of organic amendments to soils. Large quantities of organic amendments, such as manure compost, biosolid and municipal solid wastes are used as a source of nutrients and also as a conditioner to improve the physical properties and fertility of soils. These organic amendments that are low in metal(loid)s can be used as a sink for reducing the bioavailability of metal(loid)s in contaminated soils and sediments through their effect on the adsorption, complexation, reduction and volatilization of metal(loid)s. This review examines the mechanisms for the enhanced bioremediation of metal(loid)s by organic amendments and discusses the practical implications in relation to sequestration and bioavailability of metal(loid)s in soils.

Biochar reduces the bioavailability and phytotoxicity of heavy metals

Background and aimsBiochar has attracted research interest due to its ability to increase the soil carbon pool and improve crop productivity. The objective of this study was to evaluate the metal immobilizing impact of chicken manure- and green waste-derived biochars, and their effectiveness in promoting plant growth.MethodsThe immobilization and phytoavailability of Cd, Cu and Pb was examined using naturally contaminated shooting range and spiked soils. Biochar samples prepared from chicken manure and green waste were used as soil amendments.ResultsApplication of biochar significantly reduced NH4NO3 extractable Cd, Cu and Pb concentrations of soils, indicating the immobilization of these metals. Chicken manure-derived biochar increased plant dry biomass by 353 and 572% for shoot and root, respectively with 1% of biochar addition. This might be attributed to reduced toxicity of metals and increased availability of nutrients such as P and K. Both biochars significantly reduced Cd, Cu and Pb accumulation by Indian mustard (Brassica juncea), and the reduction increased with increasing amount of biochar application except Cu concentration. Metal sequential fractionation data indicated that biochar treatments substantially modified the partitioning of Cd, Cu and Pb from the easily exchangeable phase to less bioavailable organic bound fraction.ConclusionsThe results clearly showed that biochar application was effective in metal immobilization, thereby reducing the bioavailability and phytotoxicity of heavy metals.

Chromium Contamination and Its Risk Management in Complex Environmental Settings

Contamination of terrestrial and aquatic ecosystems by arsenic (As) is a very sensitive environmental issue due to its adverse impact on human health. Although not anthropogenic in origin, the problem of As contamination in groundwaters of West Bengal (India) and Bangladesh has been considered of calamitous proportion because significant segment of the population is at high risk, with untold numbers already suffering from irreversible effects of As poisoning. Elsewhere, indiscriminate disposal of industrial and mining wastes has led to extensive contamination of lands, thereby exacerbating the potential for food chain contamination. With greater public awareness of As poisoning in animal and human nutrition, there has been a growing interest in developing regulatory guidelines and remediation technologies for mitigating As-contaminated ecosystems. Although the immediate needs revolve around the stripping of As from domestic water supplies as exemplified by the affected areas in Bangladesh and West Bengal, a remediation scheme should also be explored to be able to cope with pivotal needs to abate the contamination of soils, sediments, and water and the potential to compromise the quality of the food chain. A range of technologies, including bioremediation, has been applied with varying levels of success either to remove As from the contaminated medium or to reduce its biotoxicity. This review provides general overview of the various biogeochemical processes that regulate As bioavailability to organisms, including microbes, plants, animals and humans. In turn, the role of the source term, chemical form, and chemical species of As are discussed as an overture to As bioavailability. Having laid the fundamental mechanisms and factors regulating As bioavailability, we then assembled the various physical, chemical, and biological mitigative methods that have been demonstrated, some being practical, highlighting their special strengths and potential for more effective and economical widespread applications. Because of the complexity involved in dealing with contaminated sites, exacerbated by site characteristics, nature of hydrogeology, source term, chemical form, land use, and so on, no one remedial technology might suffice. Therefore, we have attempted to offer an “integrated” approach of employing a combination of technologies at multiscalar levels, depending on extenuating circumstance, with the aim of securing viable methods, economically and technologically. Future research needs, especially in the area of As bioavailability and remediation strategies, are identified.

The influence of biochar and black carbon on reduction and bioavailability of chromate in soils.

The widespread use of chromium (Cr) has a deleterious impact on the environment. A number of pathways, both biotic and abiotic in character, determine the fate and speciation of Cr in soils. Chromium exists in two predominant species in the environment: trivalent [(Cr(III)] and hexavalent [Cr(VI)]. Of these two forms, Cr(III) is nontoxic and is strongly bound to soil particles, whereas Cr(VI) is more toxic and soluble and readily leaches into groundwater. The toxicity of Cr(VI) can be mitigated by reducing it to Cr(III) species. The objective of this study was to examine the effect of organic carbon sources on the reduction, microbial respiration, and phytoavailability of Cr(VI) in soils. Organic carbon sources, such as black carbon (BC) and biochar, were tested for their potential in reducing Cr(VI) in acidic and alkaline contaminated soils. An alkaline soil was selected to monitor the phytotoxicity of Cr(VI) in sunflower plant. Our results showed that using BC resulted in greater reduction of Cr(VI) in soils compared with biochar. This is attributed to the differences in dissolved organic carbon and functional groups that provide electrons for the reduction of Cr(VI). When increasing levels of Cr were added to soils, both microbial respiration and plant growth decreased. The application of BC was more effective than biochar in increasing the microbial population and in mitigating the phytotoxicity of Cr(VI). The net benefit of BC emerged as an increase in plant biomass and a decrease in Cr concentration in plant tissue. Consequently, it was concluded that BC is a potential reducing amendment in mitigating Cr(VI) toxicity in soil and plants.

Stabilization of carbon in composts and biochars in relation to carbon sequestration and soil fertility

There have been increasing interests in the conversion of organic residues into biochars in order to reduce the rate of decomposition, thereby enhancing carbon (C) sequestration in soils. However energy is required to initiate the pyrolysis process during biochar production which can also lead to the release of greenhouse gasses. Alternative methods can be used to stabilize C in composts and other organic residues without impacting their quality. The objectives of this study include: (i) to compare the rate of decomposition among various organic amendments and (ii) to examine the effect of clay materials on the stabilization of C in organic amendments. The decomposition of a number of organic amendments (composts and biochars) was examined by monitoring the release of carbon-dioxide using respiration experiments. The results indicated that the rate of decomposition as measured by half life (t(1/2)) varied between the organic amendments and was higher in sandy soil than in clay soil. The half life value ranged from 139 days in the sandy soil and 187 days in the clay soil for poultry manure compost to 9989 days for green waste biochar. Addition of clay materials to compost decreased the rate of decomposition, thereby increasing the stabilization of C. The half life value for poultry manure compost increased from 139 days to 620, 806 and 474 days with the addition of goethite, gibbsite and allophane, respectively. The increase in the stabilization of C with the addition of clay materials may be attributed to the immobilization of C, thereby preventing it from microbial decomposition. Stabilization of C in compost using clay materials did not impact negatively the value of composts in improving soil quality as measured by potentially mineralizable nitrogen and microbial biomass carbon in soil.

Phosphorus–arsenic interactions in variable-charge soils in relation to arsenic mobility and bioavailability

Phosphorus (P) influences arsenic (As) mobility and bioavailability which depends on the charge components of soil. The objective of this study was to examine P-As interaction in variable-charge allophanic soils in relation to P-induced As mobilization and bioavailability. In this work, the effect of P on arsenate [As(V)] adsorption and desorption was examined using a number of allophanic and non-allophanic soils which vary in their anion adsorption capacity. The effect of P on As uptake by Indian mustard (Brassica juncea L.) plants was examined using a solution culture, and a soil plant growth experiment involving two As-spiked allophanic and non-allophanic soils which vary in their anion adsorption capacity, and a field As-contaminated sheep dip soil. Arsenate adsorption increased with an increase in the anion adsorption capacity of soils. The addition of P resulted in an increase in As desorption, and the effect was more pronounced in the case of allophanic soil. In the case of both As-spiked soils and field contaminated sheep-dip soil, application of P increased the desorption of As, thereby increasing its bioavailability. The effect of P on As uptake was more pronounced in the high anion adsorbing allophanic than low adsorbing non-allophanic soil. In the case of solution culture, As phytoavailability decreased with increasing concentration of P which is attributed to the competition of P for As uptake by roots. While increasing P concentration in solution decreased the uptake of As, it facilitated the translocation of As from root to shoot. The net effect of P on As phytoavailability in soils depends on the extent of P-induced As mobilization in soils and P-induced competition for As uptake by roots. The P-induced mobilization of As could be employed in the phytoremediation of As-contaminated sites. However, care must be taken to minimize the leaching of As mobilized through the P-induced desorption, thereby resulting in groundwater and off site contamination.

Chapter two: Chromium contamination and its risk assessment in complex environmental settings

Contamination of terrestrial and aquatic ecosystems by arsenic (As) is a very sensitive environmental issue due to its adverse impact on human health. Although not anthropogenic in origin, the problem of As contamination in groundwaters of West Bengal (India) and Bangladesh has been considered of calamitous proportion because significant segment of the population is at high risk, with untold numbers already suffering from irreversible effects of As poisoning. Elsewhere, indiscriminate disposal of industrial and mining wastes has led to extensive contamination of lands, thereby exacerbating the potential for food chain contamination. With greater public awareness of As poisoning in animal and human nutrition, there has been a growing interest in developing regulatory guidelines and remediation technologies for mitigating As-contaminated ecosystems. Although the immediate needs revolve around the stripping of As from domestic water supplies as exemplified by the affected areas in Bangladesh and West Bengal, a remediation scheme should also be explored to be able to cope with pivotal needs to abate the contamination of soils, sediments, and water and the potential to compromise the quality of the food chain. A range of technologies, including bioremediation, has been applied with varying levels of success either to remove As from the contaminated medium or to reduce its biotoxicity. This review provides general overview of the various biogeochemical processes that regulate As bioavailability to organisms, including microbes, plants, animals and humans. In turn, the role of the source term, chemical form, and chemical species of As are discussed as an overture to As bioavailability. Having laid the fundamental mechanisms and factors regulating As bioavailability, we then assembled the various physical, chemical, and biological mitigative methods that have been demonstrated, some being practical, highlighting their special strengths and potential for more effective and economical widespread applications. Because of the complexity involved in dealing with contaminated sites, exacerbated by site characteristics, nature of hydrogeology, source term, chemical form, land use, and so on, no one remedial technology might suffice. Therefore, we have attempted to offer an “integrated” approach of employing a combination of technologies at multiscalar levels, depending on extenuating circumstance, with the aim of securing viable methods, economically and technologically. Future research needs, especially in the area of As bioavailability and remediation strategies, are identified.

Differential effect of biochar upon reduction-induced mobility and bioavailability of arsenate and chromate

Heavy metals such as chromium (Cr) and arsenic (As) occur in ionic form in soil, with chromate [Cr(VI)] and arsenate As(V) being the most pre-dominant forms. The application of biochar to Cr(VI) and As(V) spiked and field contaminated soils was evaluated on the reduction processes [(Cr(VI) to Cr(III)] and [As(V) to As(III))], and subsequent mobility and bioavailability of both As(V) and Cr(VI). The assays used in this study included leaching, soil microbial activity and XPS techniques. The reduction rate of As(V) was lower than that of Cr(VI) with and without biochar addition, however, supplementation with biochar enhanced the reduction process of As(V). Leaching experiments indicated Cr(VI) was more mobile than As(V). Addition of biochar reversed the effect by reducing the mobility of Cr and increasing that of As. The presence of Cr and As in both spiked and contaminated soils reduced microbial activity, but with the addition of biochar to these soils, the microbial activity increased in the Cr(VI) contaminated soils, while it was further decreased with As(V) contaminated soils. The addition of biochar was effective in mitigating Cr toxicity by reducing Cr(VI) to Cr(III). In contrast, the conversion process of As(V) to As(III) hastened by biochar was not favourable, as As(III) is more toxic in soils. Overall, the presence of functional groups on biochar promotes reduction by providing the electrons required for reduction processes to occur as determined by XPS data.

Comparative Sorption and Mobility of Cr(III) and Cr(VI) Species in a Range of Soils: Implications to Bioavailability

The sorption of chromium (Cr) species to soil has become the focus of research as it dictates the bioavailability and also the magnitude of toxicity of Cr. The sorption of two environmentally important Cr species [Cr(III) and Cr(VI)] was examined using batch sorption, and the data were fitted to Langmuir and Freundlich adsorption isotherms. The effects of soil properties such as pH, CEC, organic matter (OM), clay, water-extractable SO42– and PO43–, surface charge, and different iron (Fe) fractions of 12 different Australian representative soils on the sorption, and mobility of Cr(III) and Cr(VI) were examined. The amount of sorption as shown by Kf was higher for Cr(III) than Cr(VI) in all tested soils. Further, the amount of Cr(III) sorbed increased with an increase in pH, CEC, clay, and OM of soils. Conversely, the chemical properties of soil such as positive charge and Fe (crystalline) had a noticeable influence on the sorption of Cr(VI). Desorption of Cr(VI) occurred rapidly and was greater than desorption of Cr(III) in soils. The mobility of Cr species as estimated by the retardation factor was higher for Cr(VI) than for Cr(III) in all tested soils. These results concurred with the results from leaching experiments which showed higher leaching of Cr(VI) than Cr(III) in both acidic and alkaline soils indicating the higher mobility of Cr(VI) in a wide range of soils. This study demonstrated that Cr(VI) is more mobile and will be bioavailable in soils regardless of soil properties and if not remediated may eventually pose a severe threat to biota.

Microbial Transformation of Trace Elements in Soils in Relation to Bioavailability and Remediation

The term “trace elements” generally includes elements (both metals and metalloids) that occur in natural and perturbed environments in small amounts and that, when present in sufficient bioavailable concentrations, are toxic to living organisms (Adriano 2001). This group includes both biologically essential [e.g., cobalt (Co), copper (Cu), chromium (Cr), manganese (Mn), and zinc (Zn)] and nonessential [e.g., cadmium (Cd), lead (Pb), and mercury (Hg)] elements. The essential elements (for plant, animal, or human nutrition) are required in low concentrations and hence are known as “micro nutrients.” The nonessential elements are phytotoxic and/or zootoxic and are widely known as “toxic elements” (Adriano 2001). Both groups are toxic to plants, animals, and/or humans at exorbitant concentrations (Alloway 1990; Adriano 2001). Heavy metal(loid)s, which include elements with an atomic density greater than 6 g cm#3 [with the exception of arsenic (As), boron (B), and selenium (Se)] are also considered to be trace elements.