Arsenic removal from the contaminated soil by bio-based washing agent in a combined mechanical agitation and ultrasonication process

Abstract

A significant amount of arsenic (As) pollution has been found globally, with several health consequences. Soil washing is a cost-effective mix of chemical and physical procedures that have been used to remove heavy metals from polluted soil. This study investigated the possibility of using a lipopeptide biosurfactant from Bacillus subtilis GY19 as a washing agent to remove As from contaminated soil. To improve the effectiveness of biosurfactants, sodium carbonate and ethylenediaminetetraacetic acid (EDTA) were employed as builders to minimize the influence of exchangeable cations in the soil. The agricultural soil utilized in the study was spiked with As at an initial concentration of 418.7 mg/kg. Lipopeptide solutions were made in a series based on their critical micelle concentration (CMC). The maximal As removal capability of 5x CMC lipopeptide with 0.01 M EDTA solution was 46.1±0.9 %, while 0.01 M EDTA alone and DI water removed only 34.1±1.4 % and 16.8±0.4% As, respectively. The As removing efficiency of lipopeptide-EDTA formulation was increased with the implementation of the combined soil washing process with mechanical agitation and ultrasonication. The highest arsenic removal efficiency of 71.45±3.6% was recorded when the 1x CMC lipopeptide + 0.01 M EDTA washing agent was applied to the combination of mechanical agitation and ultrasonication washing process. The As removing efficiency was higher than the control synthetic surfactant and the soil pH of the washed soil was in the acceptable range for the agricultural standards. To optimize the soil washing condition, the Design of Experiment (DOE) was conducted through Box-Behnken design by varying the washing pH, washing time, and the solid: liquid ratio. Two soil samples with the initial concentrations of 434 mg/kg as low As concentrated soil and 2,572 mg/kg as high As concentrated soil were investigated. The maximum As removal efficiency for lower As concentrated soil was 80%, which was obtained by using pH 4.0 washing agent, 1.0 solid: liquid ratio, and 40 minutes washing time. For the higher As concentrated soil, the maximum washing efficiency of 86% was obtained from pH 7.1 washing agent, 0.1 solid: liquid ratio, and 20 minutes washing time. The results indicated that the mixture of lipopeptide and EDTA could be applied to wash As contaminated soil by the combination of mechanical agitation and ultrasonication process. However, the scale-up experiments should be investigated to confirm the efficiency of lipopeptide+EDTA washing agent and the combined washing process for As remediation in the soil. Also, the toxicity of the washing agent as well as recycling of the washed leachate must be studied.