A laboratory investigation of microbe-inducing CdCO<subscript>3</subscript> precipitate treatment in Cd<superscript>2+</superscript> contaminated soil.

Background, aim and scope This paper discusses a method investigating the reduction of free heavy metal cation contents in soil through the use of microbe-inducing precipitate (MIP). Materials and methods Bacteria Pasteurella is adopted to induce urea hydrolysis and produce CO 3 2- . Free Cd 2+ is then precipitated as CdCO 3 in soil. Bacteria Pasteurella reproduces and the produced urease activity is then determined to evaluate the Cd 2+ effect. Wheat seeds are planted in Cd 2+ contaminated soil after treatment with microbial inducing CdCO 3 precipitate. Wheat-growing status is photographed and wheat stem length data are recorded to evaluate the microbial treatment. Results Bacteria Pasteurella is discovered to consistently reproduce in 10 mg L -1 Cd 2+ solution for over 120 h. The optimal result is a free Cd 2+ concentration reduction by 92% after a 28-day microbial treatment on Cd 2+ contaminated soil (initial Cd 2+ content 10 μg g -1 (soil)). Discussion Free Cd 2+ in soil is immobilized by the microbe-inducing CdCO 3 precipitate. The mechanism goes through the stages of hydrolyzing CO(NH 2 ) 2 into CO 3 2- by Pasteurella bacteria's enzymolysis and reducing free Cd 2+ content in soil by immobilizing free Cd 2+ in the CdCO 3 precipitate. Based on the Michaelis-Menten equation of enzyme dynamics, urease constant K m is 1,230 mol L -1 and maximum urease catalytic rate V max is 4.55 g L -1 h -1 . Urease catalytic rate V is no less than 0.94 g L -1 h -1 in 10 μg g -1 Cd 2+ contaminated soil in 72 h under repetitious urea supplement (40 mg L -1 per 24 h). Before the wheat tassel stage, the average wheat stem length cultivated in soil samples T2, T3, and T4 varies from 16 to 22 cm. Part of the stem length of T3 and T4 is well over 22 cm. The specific Cd 2+ treatment method used in T4 is the best for rehabilitating Cd 2+ contaminated soil in the investigation. Conclusions The treatment is based on the mechanism of hydrolyzing CO(NH 2 ) 2 into CO 3 2- by Pasteurella bacteria's enzymolysis and reducing free Cd 2+ content in soil by immobilizing free Cd 2+ in CdCO 3 precipitate. Based on the Michaelis-Menten equation of enzyme dynamics, Cd 2+ is observed to hold back Pasteurella-producing urease activity in solution and in soil. But Pasteurella bacteria are also endurable to Cd 2+ with an appropriate concentration. Cd 2+ treatments significantly reduced the residual contents of free Cd 2+ in soil. Recommendations and perspectives The microbe-inducing carbonate precipitate method used in this investigation is patented by China Patent Bureau in 2007. It is recommended in application for its low-cost, repeatable and environmentally friendly specialties in China. [ABSTRACT FROM AUTHOR]

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