Environ Pollut. 2021 Oct 9:118323. doi: 10.1016/j.envpol.2021.118323. Online ahead of print.
Hydroxyapatite (HAP) can effectively immobilize soil heavy metals, but excess phosphate would be released to aquatic ecosystem, resulting in eutrophication. This study investigated the effects of ferrihydrite (FH) on the HAP immobilization of copper (Cu) and cadmium (Cd) and their reduction of phosphorus release under flooding-drainage alternation conditions. Results showed that the incorporation of HAP and FH significantly increased soil solution pH and decreased Cu2+ and Cd2+ concentrations. Applications of FH, HAP, and FH-HAP (FH and HAP combination) can all enhance soil pH and reduce CaCl2-extractable and exchangeable Cu and Cd, but HAP addition increased soluble phosphate by 6.60-7.77 times compared to control. However, FH-HAP application can significantly reduce phosphate release by 92.7-99.7% compared to HAP application. FH-HAP was the most effective to reduce exchangeable Cu and Cd by 49.8-93.4% and 50.9-88.8% and decreased labile and moderately labile phosphorus by 34.0-74.4% and 13.5-18.6%, respectively, while increased stable phosphorus by 22-45.1% than single HAP. All FH treatments significantly increased amorphous iron oxides by the factors of 4.66-20.8, but only 3% and 5% of FH applications slightly enhanced crystal iron oxides by the factors of 0.81-1.27. The major implication is that the combination of FH and HAP can not only immobilize of Cu and Cd, but also reduce the risk of phosphate release by HAP addition.
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