Effects of nanometer magnesium hydroxide on soil cadmium form under different cadmium levels

Cd is one of the most serious and harmful heavy metals in the world. Nanomaterials as a new passivating agent for heavy metals in soil have become a hot issue. But nanometer magnesium hydroxide application to heavy metal pollution in soils has not been reported. The laboratory culture experiments were conducted to study the effect of nanometer magnesium hydroxide and ordinary magnesium hydroxide (100, 200, and 300 mg kg(-1)) as cadmium passivators on soil cadmium form under different cadmium levels (1, 5, 10, and 15 mg kg(-1)). The results showed that the soil Cd distribution ratio of exchangeable Cd (EX-Cd FDC) was 66.7% to 81.8% at cadmium levels of 1, 5, 10, and 15 mg kg(-1), identifying it as the main form of soil cadmium. Soil EX-Cd FDC reached the lowest level on the 14th day of culture. Addition of both nanometer magnesium hydroxide and ordinary magnesium hydroxide decreased the EX-Cd FDC of the soil at each cadmium level. With increasing amount, the soil EX-Cd FDC followed a decreasing trend. During the 0-28 days of culture, at the levels of 1, 5, 10, and 15 mg kg(-1) cadmium, the EX-Cd FDC of soil treated with nanometer magnesium hydroxide and ordinary magnesium hydroxide decreased by 11.4% to 67.7%, 7.8% to 37.2%, 7.7% to 36.4%, and 5.0% to 28.8% (nanometer magnesium hydroxide) and .5% to 49.5%, .6% to 15.0%, 1.0% to 18.1%, and .7% to 14.6% (common magnesium hydroxide), respectively, compared with control. Under the same application amount, nanometer magnesium hydroxide achieved a better effect than ordinary magnesium hydroxide in reducing soil EX-Cd FDC. The antagonism between Cd and Mg competes for exchange sites during soil adsorption or Mg2+ forms hydroxy-magnesium through OH- coordination in the soil via hydrolysis equilibrium. This facilitates the formation of precipitates, such as carbonates in Cd2+, and reduces active-state cadmium in soil. The effect of nanometer magnesium hydroxide in passivating active cadmium on cadmium-contaminated soil exceeds that of common magnesium hydroxide.