Utilization of waste materials in remineralising Indian soils: A geochemical study

Large portions of the earth's landmass are poorly suited to agriculture because their soil properties barely support plant growth. This is especially true of the lateritic soils, vertisols and sodic-alkaline soils of the Indian subcontinent. It has been suggested that addition of specific solid waste materials to these soils will improve their physical and chemical properties so that they may be used to grow economically valuable crops. The risks associated with these additions, for instance, concurrent increase in total and bioavailable concentrations of toxic trace elements, can only be assessed if the amending material and the soils are completely characterized before and after amendments and monitor changes for several years following treatments. In this study the geochemistry and mineralogy of the above mentioned soils from two states in India were determined to assess the effect of waste materials (coal ash-sewage sludge-weed) in regenerating marginal-degraded soils. Assemblages of goethite, hematite, kaolinite and quartz in lateritic soils, quartz, smectite, vermiculite, illite, chlorite and feldspars in vertisols, trona, muscovite, calcite and quartz in sodic-alkaline soils were identified using X-ray diffraction, electron probe microanalysis, scanning electron microscopy and micromorphological studies. Total and bioavailable concentrations of bioessential nutrients and selected trace elements (As, Co, Cr, Ni, Zn, Pb and Cu) in the soils (before and after amendments) and waste materials were determined by aqua regia and Mehlich extractions using Inductively Coupled Plasma Optical Emission Spectroscopy. Sequential chemical extractions were followed to assess semi-quantitatively the fractionation of the trace elements in the different fractions of the soils (exchangeable, specifically adsorbed, Fe-Mn oxyhydroxides, organic/sulphides and residual). X-ray Photoelectron Spectroscopy was used briefly to evaluate surface availability of elements. The results show improved soil characteristics such as pH and organic matter content as a result of amendments. In addition there is an enhanced bioavailability of essential plant nutrients while the bioavailability of toxic species remained unchanged, or declined, without breaching the international guidelines for biosolid use for agriculture. It is concluded that while adverse changes may occur in some cases this study suggests that the amendments did not pose an environmental risk but did substantially improve bioproductivity.