Efficacy Assessment of Amended Laterite Soil as a Subsurface Liner to Attenuate Migration of Contaminants in Leachate of Ash Pond Structures

Abstract

Coal-based Thermal Power Plants (TPP) have been a prime source of generation of power in India for the last several decades. Almost 65% of the total electricity generated in India comes from TPP, and 85% of all the TPPs use coal as their fuel. Indian coal being low grade, have an ash content of about 40–45%, which is relatively high in comparison to imported coals (10–15%). Thus, a large amount of ash is being produced daily, which not only is one of the significant environmental threats to air, water & soil but also requires large areas of land for its disposal. The fly ash produced are disposed of in the form of slurry in ash ponds. There has been a noteworthy increase in the utilization of fly ash over the last few years, in road sub-base, construction materials, building engineering, backfilling and agriculture. However, the disposal of fly ash remains a concern for TPP in India. The various heavy metals (Cd, Zn, Ni) present in the leachate tend to contaminate the sub-surface soil and groundwater and, consequently, increase the toxicity of the surrounding ecosystem. The earlier researches are mainly confined to the assessment of efficiency of either pure laterite soil or laterite bentonite mixture to be used as liner materials in waste containment structures. However, it is also reported elsewhere in the literatures that the usage of bentonite alone not only decreases the permeability but also decreases the shear strength, which is a very essential factor in catering the overburden pressure of ash ponds. The present study is undertaken to assess the effectiveness of locally available Lateritic soil (L.S.) modified with commercially procured Bentonite (B) & Fly Ash (F.A.) as liner content in the construction of Ash Pond. Because of the high permeability of ~10−8 m/s and the moderate shear strength, lateritic soil is adjusted in a selected proportion with commercially procured Bentonite and Fly Ash, i.e. 8 (L.S.): 1 (B): 1 (F.A.) to determine its pollutants which attenuate the potential for use as a liner material in ash pond structure to limit sub-surface migration of heavy metals present in ash leachate. It is observed that in the amended soil, plasticity, hydraulic conductivity, swelling and shrinkage properties decreases, and the shear strength and dry unit weight increases with the increase in fly ash content. The concentrations of the Cadmium is observed to be in the range of 0.127–0.37 ppm. In the case of Nickel, it is found to be lying between 0.037 and 0.332 ppm, while for Zinc, it is observed to be within the range of 0.004–0.215 ppm. Batch adsorption test results indicate metal removal efficiency of amended soil about 96, 98, 97%, respectively for cadmium, nickel and zinc. In this study, the Linear, Langmuir, Freundlich and Temkin isotherms are tested for analysing the equilibrium data. Langmuir proved to be the best-fit with R2 values of 0.981 and 0.9759, respectively. The findings of the fixed-bed column experiment also show isotherm for Cadmium with a regression coefficient (R2) value of 0.9739, while for Nickel and Zinc, it follows Temkin isotherm excellent potential for heavy metal adsorption in modified laterite soil. The predicted breakthrough curves (BTCs) using the HYDRUS 1D finite-difference transport software package well corroborates experimental findings with regression coefficients of 0.98–0.99. The design life of liner made with amended laterite soil increased significantly in comparison with the non-amended laterite soil for the same liner thickness. Therefore, amended lateritic soil can be considered as a promising adsorbent for the arrest of heavy metal pollutants present in the leachate of ash ponds as well as an excellent candidate for primary liner material in waste containment structures to restrict the migration of pollutants to the aquifer and thereby protecting the precious groundwater from contamination.