This study evaluated the efficacy of enzyme induced calcite precipitation (EICP) in restricting the mobility of heavy metals in soils. EICP is an environmentally friendly method that has wide ranging applications in the sustainable development of civil infrastructure. The study examined the desorption of three heavy metals from treated and untreated soils using ethylene diamine tetra-acetic acid (EDTA) and citric acid (C6H8O7) extractants under harsh conditions. Two natural soils spiked with cadmium (Cd), nickel (Ni), and lead (Pb) were studied in this research. The soils were treated with three types of enzyme solutions (ESs) to achieve EICP. A combination of urea of one molarity (M), 0.67 M calcium chloride, and urease enzyme (3 g/L) was mixed in deionized (DI) water to prepare enzyme solution 1 (ES1); non-fat milk powder (4 g/L) was added to ES1 to prepare enzyme solution 2 (ES2); and 0.37 M urea, 0.25 M calcium chloride, 0.85 g/L urease enzyme, and 4 g/L non-fat milk powder were mixed in DI water to prepare enzyme solution 3 (ES3). Ni, Cd, and Pb were added with load ratios of 50 and 100 mg/kg to both untreated and treated soils to study the effect of EICP on desorption rates of the heavy metals from soil. Desorption studies were performed after a curing period of 40 days. The curing period started after the soil samples were spiked with heavy metals. Soils treated with ESs were spiked with heavy metals after a curing period of 21 days and then further cured for 40 days. The amount of CaCO3 precipitated in the soil by the ESs was quantified using a gravimetric acid digestion test, which related the desorption of heavy metals to the amount of precipitated CaCO3. The order of desorption was as follows: Cd > Ni > Pb. It was observed that the average maximum removal efficiency of the untreated soil samples (irrespective of the load ratio and contaminants) was approximately 48% when extracted by EDTA and 46% when extracted by citric acid. The soil samples treated with ES2 exhibited average maximum removal efficiencies of 19% and 10% when extracted by EDTA and citric acid, respectively. It was observed that ES2 precipitated a maximum amount of calcium carbonate (CaCO3) when compared to ES1 and ES3 and retained the maximum amount of heavy metals in the soil by forming a CaCO3 shield on the heavy metals, thus decreasing their mobility. An approximate improvement of 30% in the retention of heavy metal ions was observed in soils treated with ESs when compared to untreated soil samples. Therefore, the study suggests that ESs can be an effective alternative in the remediation of soils contaminated with heavy metal ions.

Soil treatment methods to cope with ever-growing demands of construction industry and environmental aspects are always explored for their suitability in different in-situ conditions. Of late, enzyme induced calcite precipitation (EICP) is gaining importance as a reliable technique to improve soil properties and for contaminant remediation scenarios. In the present work, swelling and permeability characteristics of two native Indian cohesive soils (Black and Red) are explored. Experiments on the sorption and desorption of multiple heavy metals (Cd, Ni and Pb) onto these soils were conducted to understand the sorptive response of the heavy metals. To improve the heavy metal retention capacity and enhance swelling and permeability characteristics, the selected soils were treated with different enzyme solutions. The results revealed that EICP technique could immobilize the heavy metals in selected soils to a significant level and reduce the swelling and permeability. This technique is contaminant selective and performance varies with the nature and type of heavy metal used. Citric acid (C6H8O7) and ethylene diamine tetra-acetic acid (EDTA) were used as extractants in the present study to study the desorption response of heavy metals for different EICP conditions. The results indicate that calcium carbonate (CaCO3) precipitate deposited in the voids of soil has the innate potential in reducing the permeability of soil up to 47-fold and swelling pressure by 4-fold at the end of 21 days of curing period. Reduction in permeability and swell, following EICP treatment can be maintained with one time rinsing of the treated soil in water to avoid dissolution of precipitated CaCO3. Outcomes of this study have revealed that EICP technique can be adopted on selected native soils to reduce swelling and permeability characteristics followed by enhanced contaminant remediation enabling their potential as excellent landfill liner materials.

In this study, the relative performance of two soils after amending them with suitable matrices, in retaining the heavy metal ions has been evaluated. Al-Ghat soil and Al-Qatif soil of Saudi Arabia were mixed with different percentages of lime. Heavy metal ions such as copper and chromium were used to carry out sorption studies. Experimental data was used to plot adsorption isotherms. Langmuir isotherm was found to be more suitable than Freundlich isotherm for all soil mixtures. It can be concluded that Al-Qatif soil lime mixture can be used as a filter material and Al-Ghat soil lime mixture as the main liner material to attenuate selected heavy metals.

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Soils are known to retain heavy metals through a number of processes. In this study, four different leaching methods viz. ASTM D3987, toxicity characteristics leaching procedure (TCLP), extended TCLP, and caged TCLP have been used to study the leaching behavior of Cd 2? , Ni 2? and Pb 2? on red and black cotton soils. Further, these soils were amended with nano calcium silicate (NCS). Also, sequential extraction was performed on these soil mixtures to know the extent of sorption taking place in different phases of soils such as exchangeable, oxidizing, reducing, organic content and residual phases. It was found that as per ASTM D3987, all three heavy metals with red soil and black cotton soil were retained well in the short term, and over a long-term period, the metal ion was released back. When amended with NCS, both red soil and black cotton soil exhibited satisfactory retention levels over long periods which proved that NCS was responsible for encapsulation of metal ions in its matrix. To investigate the retention efficiency, extensive leaching tests were performed, and it was found that as per TCLP, extended TCLP and caged TCLP tests, the retention with NCS-amended red earth soil was 88, 90 and 94 % compared to untreated case. Sequential extraction proved that encapsulation was durable in NCS-amended soils compared to untreated ones.Hence, it is concluded that NCS-amended soil is a prospective material to attenuate heavy metals taking into consideration their long-term encapsulation effect in considered soils. This soil NCS mixture is expected to be a potent agent for in situ treatment of brown fields and an excellent substitute for existing conventional landfill liner materials.