Adaption And Improvement Of EICP Technology For Stabilization Of Cadmium-contaminated Calcareous Soil Under Wastewater Irrigation

Enzyme-induced carbonate precipitation（EICP）technology is new-emerging biomineralization technology relying on hydrolysis of urea and generation of carbonate ions(CO₃^2-).Then calcium carbonate（CaCO₃）was precipitated while calcium ions(Ca²⁺)was present.Long-term wastewater irrigation leads to a decrease of CaCO₃ in topsoil and the activation of soil inherent cadmium（Cd）.In this paper,we selected Cd-contaminated calcareous soil under long-term wastewater irrigation and set Cd as the characteristic contaminant.A simulating lab experiment was designed to determine the capacity of conventional and modified EICP technology for restoring calcium,stabilization of Cd in wastewater-irrigated calcareous soil and potential environmental risks.On this basis,we conducted field experiment and combined environmental impact analysis to reveal the influencing mechanism of modified EICP technology on environmental behavior of Cd in soil-corn system in long-term wastewater-irrigated farmland.Finally,we evaluated ecological and health risk of Cd in soil-corn system.The following conclusions were obtained from this study:（1）There existed environmental risks of soil acidification and salinization,non-point source pollution due to the addition of exogenous Ca²⁺in conventional EICP process.Meantime,the addition of free Ca²⁺increased soil bioavailable Cd from 0.81mg kg^-1 to 17.18 mg kg^-1.This indicated that conventional EICP technology was not suitable for stabilization of Cd in long-term wastewater-irrigated calcareous soil.Under modified EICP treatment,the increase of soil pH and co-precipitation of Cd²⁺with CaCO₃ led to a transformation of inherent Cd species,from exchangeable Cd to carbonate,oxide,residual Cd,and the decrease in soil Cd bioavailability.The lowest bioavailable Cd concentration was observed under the treatment of high amount of urease and urea,at 0.17 mg kg^-1 and reducing by 80.5%.Therefore,modified EICP technology effectively stabilized soil Cd.（2）Modified EICP process can restore the content of CaCO₃ in topsoil,and increase soil clay and dissolved organic carbon（DOC）content under field conditions.Nitrogen fertilizer promotes the formation of soil Cd-DOC complex.The mobilization of soil Cd by Cd-DOC was stronger than the decrease in soil Cd bioavailability by modified EICP technology.However,under the chelation of soil Cd-DOC and the bound of hydroxyl groups in the epidermal cell of corn root,soil Cd was enriched and fixed Cd in corn root,thus reducing Cd translocation from root to shoot.Modified EICP can promote the improvement of soil mechanical composition and the accumulation of corn dry matter,while increase corn grain yield by 18.1%at the highest and ensure that Cd concentration in corn kernel was lower than the 0.1 mg kg^-1 maximum level in the cereal set by China’s Food Safety National Standard.Hence,modified EICP technology is suitable for the remediation of wastewater-irrigated calcareous farmland.（3）Modified EICP technology reduced the toxicity and bio accessibility of Cd in field soil by the cation exchange adsorption of soil colloids,the formation of aluminosilicate and cementing bridging effect of CaCO₃.The TCLP-extracted Cd concentration was reduced by up to 79.5%(from 1.76 mg kg^-1 to 0.36 mg kg^-1).Cd in soil-corn system in demonstration field posed no significant non-carcinogenic risk for adults and children,but does pose a carcinogenic risk through soil ingestion and food consumption.Improvement of agricultural planting structures,adjusting dietary structure,processing corn grains and corncobs,and exploiting corn root and stem resources can be conducted to realize the safe utilization of wastewater-irrigated calcareous farmland and agricultural sustainable development.In general,modified EICP technology shows potential for restoring calcium and reducing the bioavailability of Cd in wastewater-irrigated calcareous soil without environmental risks.Under field conditions,modified EICP technology ensured food security and food safety,and effectively reduced the toxicity and bioaccessibility of Cd in soil-corn system.Modified EICP technology is suitable for in-situ stabilization of Cd in calcareous soil under long-term wastewater irrigation.