Biopolymer-assisted enzyme-induced carbonate precipitation for immobilizing Cu ions in aqueous solution and loess

Wastewater, discharged in copper (Cu) mining and smelting, usually contains a large amount of Cu2+. Immobilizing Cu2+ in aqueous solution and soils is deemed crucial in preventing its migration into surrounding environments. In recent years, the enzyme-induced carbonate precipitation (EICP) has been widely applied to Cu immobilization. However, the effect of Cu2+ toxicity denatures and even inactivates the urease. In the present work, the biopolymer-assisted EICP technology was proposed. The inherent mechanism affecting Cu immobilization was explored through a series of test tube experiments and soil column tests. Results indicated that 4 g/L chitosan may not correspond to a higher immobilization efficiency because it depends as well on surrounding pH conditions. The use of Ca2+ not only played a role in further protecting urease and regulating the environmental pH but also reduced the potential for Cu2+ to migrate into nearby environments when malachite and azurite minerals are wrapped by calcite minerals. The species of carbonate precipitation that are recognized in the numerical simulation and microscopic analysis supported the above claim. On the other hand, UC1 (urease and chitosan colloid) and UC2 (urea and calcium source) grouting reduced the effect of Cu2+ toxicity by transforming the exchangeable state-Cu into the carbonate combination state-Cu. The side effect, induced by 4 g/L chitosan, promoted the copper-ammonia complex formation in the shallow ground, while the acidic environments in the deep ground prevented Cu2+ from coordinating with soil minerals. These badly degraded the immobilization efficiency. The Raman spectroscopy and XRD test results tallied with the above results. The findings shed light on the potential of applying the biopolymer-assisted EICP technology to immobilizing Cu ions in water bodies and sites.