Efficacy of Enzymatically Induced Calcium Carbonate Precipitation in the Retention of Heavy Metal Ions

Arif Ali Baig Moghal,Kehinde Lemboye,Mohammed Abdul Lateef,Bhaskar C S Chittoori,Abdullah Almajed,Syed Abu Sayeed Mohammed

doi:10.3390/su12177019

Abstract

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.

Highlights

Increase in population and attempts to satisfy the ever-growing demands of the same have led to industrialization, widespread construction activity, and extensive mining
The main process that leads to the formation of calcite is hydrolysis of urea by the urease enzyme into CO2 and NH3, and the speciation of NH3 leads to the development of NH4+ ions, which creates a suitable environment for are incorporated in CaCO3 crystal lattice by replacing Ca2+ ions or by creating defects on the calcium carbonate crystals, or even by penetrating the CaCO3 interstice
The formation of carbonates of heavy metals occurs in the microenvironment of the mineral carbonates, and the process is even applicable for radionuclides such as strontium forming strontium carbonate (SrCO3) [74]

Summary

Introduction

Increase in population and attempts to satisfy the ever-growing demands of the same have led to industrialization, widespread construction activity, and extensive mining. Engineered landfills, which are identified as the most viable means of landfilling solid municipal wastes, have become sources of leachates rich with high levels of toxicity, fluorides, nitrates, and heavy metals [4,5,6]. The dumping of contaminated soils into landfills has made landfill management a major challenge, as these sites become major sources of leachates rich in heavy metals, fluorides, and other contaminations. Heavy metals that are naturally present in the soil cause less damage than those that accumulate due to human activity [31]. Industries play an important role in increasing heavy metal concentrations in surface soils above permissible levels by releasing toxic fumes into the atmosphere, because traces of fumes find their way to the soil surface [33,34]

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