Different Effects And Mechanisms Of Phosphate On The Adsorption And Desorption Of Cadmium In Red Soil And Cinnamon Soil

The area of Cd-contaminated soils in our country were increasing with the industrialization development.This had a bad effect on the crop yield and quality, meanwhile, endangered human health through food chain. So it was very important to take some effective measures to repaire Cd-contaminated soils.It had been confirmed that phosphorous compound can be used as a cheap, effective chemical fixative to repaire Cd-contaminated soils,but the mechanisms of phosphate on the adsorption-desorption of cadmium in soils were not clear.The effects and mechanisms of the concentration of KH₂PO₄, three kinds of phosphate (NH₄H₂PO₄, KH₂PO₄, Ca(H₂PO₄)₂) and the ratios of P/Cd on the adsorption-desorption of cadmium in red soil and cinnamon soil were investigated by the methods of simulated tests and pot experiments. The effects of phosphates on activity and phytoavailability of Cd in red soil and cinnamon soil were also investigated. The results were shown as follows:1. The adsorption ability of Cd in cinnamon soil was stronger than in red soil.The adsorption amount of cadmium in red soil varied with the increasing concentration of KH₂PO₄ in the peak-shape-like curves; while in cinnamon soil, the adsorption amount of cadmium decreased significantly. The desorption amount of cadmium increased linearly in two soils with rising adsorption amount of cadmium.The obvious difference of desorption amount of cadmium didn't occur under the different concentration of KH₂PO₄.2. The adsorption amount of cadmium in red soil varied with the types of phosphate. The maximum amount of adsorbed cadmium in red soil decreased by 2.5%～7.9% after adding Ca(H₂PO₄)₂. The adsorption percentage of cadmium substantially declined by 10%～20% with the increasing concentration of Ca(H₂PO₄)2. Whereas, NH4H₂PO₄ and KH₂PO₄ didn't influence on the adsorption amount of cadmium in red soil. The adsorption percentage of cadmium in cinnamon soil substantially decreased with the increasing concentration of three kinds of phosphate. The obvious difference was observed under treatments of high rates of Cd and P (Ca(H₂PO₄)2>KH₂PO₄>NH₄H₂PO₄). The desorption amount and desorption percentage of cadmium from the red soil markedly decreased by 15.8%～27.8% and by 3.7%～9.6% with the increasing concentration of phosphates. The desorption amount of cadmium by Ca(H₂PO₄)2 was higher than that of NH4H₂PO₄ and KH₂PO₄ if the amount of cadmium adsorption was higher. Otherwise, the desorption percentage of cadmium by Ca(H₂PO₄)2 was higher than that of NH4H₂PO₄ and KH₂PO₄ if the amount of cadmium adsorption was lower. The desorption amount of cadmium from the cinnamon soil drastically increased by 20.8%～79.7% with the increasing concentration of phosphates under higher amount of cadmium adsorption. Only under the treatment of Cd20, the desorption amount of cadmium by Ca(H₂PO₄)2 was obviously higher than that of NH4H₂PO₄ and KH₂PO₄.3. The adsorption amount and desorption amount of cadmium significantly decreased with the increasing ratio of P/Cd. At the same ratio of P/Cd, the higher the amount of P and Cd, the more adsorption and desorption amount of cadmium.4. The soil pH value decreased by 2.4%～14.8% in Cd-polluted red soil. The pH of polluted soil significantly increased after adding three kinds of phosphate. The pH of red soil increased by 10.8%～24.8% after adding phosphates in the treatment of the third level of Cd pollution. But there was no obvious difference between different phosphates. While the pH of red soil increased by 5.2%～20.7% after adding phosphates in the treatment of second level of Cd pollution. The influencing degree of phosphates on pH increased in the order of Ca(H₂PO₄)2≈KH₂PO₄>NH4H₂PO₄. The cinnamon soil pH increased by 5.2%～11.9% after Cd pollution. The polluted soil pH significantly decreased by 1.7%～4.2% after adding NH4H₂PO₄, but significantly increased after adding KH₂PO₄ and Ca(H₂PO₄)2 under the treatment of second level of Cd pollution without nutrient.5. The available Cd content obviously increased after Cd pollution in two soils. After adding phosphates, the available Cd content in polluted red soil drastically increased by 17.0%～122.7%. The influence degree of phosphates increased in the order of Ca(H₂PO₄)2>KH₂PO₄>NH4H₂PO₄. The available Cd content in polluted cinnamon soil decreased by 2.4%～13.4% after adding NH4H₂PO₄. While after adding KH₂PO₄ and Ca(H₂PO₄)2, the available Cd content in polluted cinnamon soil increased by 3.0%～15.0%, as well as, it was higher in the trentment of Ca(H₂PO₄)2 than that of KH₂PO₄.6. The biomass of rapeseed evidently decreased by 36.2%～82.6% in Cd-polluted two soils.After adding phosphates, the biomass of rapeseed in polluted red soil drastically increased by 18.7%～291.1%, the influence degree increased: Ca(H₂PO₄)2>KH₂PO₄>NH4H₂PO₄. While in polluted cinnamon soil, the biomass of rapeseed drastically increased by 31.5%～991.2%, the influence degree indicated: Ca(H₂PO₄)2≈KH₂PO₄>NH4H₂PO₄.7. The Cd content in rapeseed remarkably increased by 8.3%～175.4% in Cd- polluted two soils. After adding phosphates to polluted red soil, the Cd content in rapeseed significantly increased by 8.3%～60.6% in the treatment of the second level of Cd pollution, but obviously decreased by 11.1%～58.4% in the treatment of the third level of Cd pollution. The Cd content in rapeseed decreased by 7.9%～19.8% in the treatment of the second level of Cd pollution in cinnamon soil after adding phosphates. Only after adding KH₂PO₄ into the third level of Cd-polluted cinnamon soil, the Cd content in rapeseed obviously increased.To sum up,by investigating the difference of the concentration of KH₂PO₄, three kinds of phosphate (NH4H₂PO₄, KH₂PO₄, Ca(H₂PO₄)2) and the ratios of P/Cd on the adsorption-desorption of cadmium in red soil and cinnamon soil, combining pot experiments, the effects of phosphates on activity and phytoavailability of Cd in red soil and cinnamon soil were compared, the conclusion could be used as a reference for repairing Cd-contaminated soils.