Remediation of Heavy Metal Contaminated Soil and Sediment And its Effective Reuse in Construction
Abstract
This research explores approaches to mitigate the environmental impact of heavy metal contamination in soil and dredged sediment. It focuses on three main areas: chemical remediation through nano zero-valent iron (nZVI), bacterial remediation techniques using Bacillus subtilis, and the stabilization process with industrial by-product ground granulated blast furnace slag (GGBS). It also includes studies of strength and leachability of both contaminated and treated soil and dredged sediment to reuse in construction applications.
The chemical remediation involving bare nZVI treatment investigates the effectiveness of nZVI in remediating heavy metals such as cadmium (Cd), nickel (Ni), and zinc (Zn) under varying environmental and operational conditions. Key factors studied include the influence of soil grain size distribution (GSD), heavy metals and their concentrations, and nZVI dosage on immobilization efficiency. Experiments were conducted with cadmium, nickel, and zinc, the toxic heavy metals found in excess in dredged sediment and soil. To understand the effect of GSD on immobilization efficiency, soils with different portions of particles finer than 75 μm were experimented with. Immobilization efficiency was obtained by comparing the leachability of contaminated and treated soil samples using the toxicity characteristic leaching procedure (TCLP) as per US environmental protection agency (USEPA 1311). Reduction in leachable fraction and variation in the speciation of heavy metals was observed through the sequential extraction procedure (SEP). The response surface methodology (RSM) was adopted for analysis with three levels of GSD, contamination, and nZVI dosage for the three heavy metals: cadmium (Cd), nickel (Ni), and zinc (Zn). The efficiency variation followed an optimum curve concerning GSD, contamination level, and the nZVI dosage. This was observed due to kaolinite and montmorillonite minerals in the soil. An interaction between these clay minerals and nZVI reduced efficiency for soils with high fine contents. Maximum efficiency of immobilization for Cd was 56.3%, for Ni 61.68%, and that for Zn was 64% for coarser soil, whereas a 5-10% decrease in immobilization efficiency was observed with an increase in finer content in the soil. For multi-metal contamination, the immobilization efficiency gradually decreased with an increase in finer soil, in the case of all the heavy metals. It was observed that for all single metal and multi-metal contamination, the interaction between soil and the nZVI and competitive behaviour of metals played a vital role in immobilization along with the factors considered. This work also considered challenges related to the interference of metals during atomic absorption spectroscopy (AAS) analysis.
The bacterial remediation approaches focused on identifying and characterizing bacterial strains capable of bio-remediating heavy metal contamination in dredged sediments and soils. The study
evaluates the effects of critical factors such as bacterial concentration, cementation reagents, calcium sources, moisture content, and incubation time on the efficiency of heavy metal immobilization on two different soils: red soil and dredged sediment. This investigation aims to facilitate the adoption of bacterial bioremediation as a sustainable and cost-effective solution for managing contaminated soils and sediments. All the experiments were designed using RSM. The bacterial concentration was varied between optical density OD600 0.8 and OD600 1.6, moisture content was varied between 12% and 100%, and incubation days were varied between 1 and 7 days. Three different sources of Calcium were experimented in cementation reagent, CaCl2, Ca(OH)2, and CaCO3. It was observed that Ca(OH)2 in cementation reagent could immobilize 99% Zn at 1 molar concentration, 96% Ni and 94% Cd in red soil and 99.8% Zn at 1M concentration, 97.2% Ni and 94.3% Cd in dredged sediment. The available or exchangeable fraction of heavy metals in soil were observed to be converted to the carbonate-bound and organic matter-bound fraction.
The reuse of contaminated sediment emphasizes transforming treated soil and dredged sediment into valuable industrial materials to minimize landfill disposal. This work explores the feasibility and safety of blending treated sediment with industrial by-products such as ground granulated blast-furnace slag (GGBS) to produce construction materials like bricks to ensure minimal environmental risks while maximizing resource recovery and economic benefits. By reducing waste generation and promoting circular economy principles, this research contributes to landfill reduction and the conservation of natural resources. The stabilization experiments were conducted on the contaminated soil, nZVI-treated soil, and bioremediated soil with different combinations of lime and GGBS to explore the optimum combination. It was observed that nZVI-treated soil could achieve slightly higher strength, whereas the bioremediated soil stabilized with lime and GGBS showed similar strength to that of the contaminated stabilized soil. Binder consisting of 10% lime and 20% GGBS, constituting 30% binder, could lead to 10.69 MPa and 9.5 MPa strength at 28 days for red soil and dredged silt, respectively. All the samples were tested for TCLP as well as the SPLP Synthetic Precipitate Leaching Procedure and were observed to have zero leachability of all the heavy metals. The findings offer practical solutions for environmental restoration while promoting the reuse of heavy metal contaminated soil and dredged sediments in industrial applications.
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