Immobilization Mechanism of Nano-Hydroxyapatite on Lead in the Ryegrass Rhizosphere Soil Under Root Confinement.

The immobilization effect and mechanism of nano-hydroxyapatite(NHAP) on Pb in the ryegrass rhizosphere soil were studied by root-bag experiment. The speciation analysis results revealed that the residual Pb concentrations in the rhizosphere soil significantly increased after NHAP application. The acid-soluble and reducible Pb concentrations significantly decreased, indicating that NHAP had obviously immobilized Pb. Meanwhile, NHAP significantly promoted the secretion of tartaric acid from ryegrass roots, resulting the rhizosphere soil pH had been below that of the control group. This helped to relieve the stress of Pb on ryegrass, also promoted the dissolution of NHAP, resulting the formation of stable precipitation with more Pb ions. NHAP increased the rhizosphere soil pH by 0.03 to 0.17, which promoted the conversion of Pb to non-utilizable bioavailability. The total Pb mass balance indicated only a very small proportion Pb transferred to the shoots through ryegrass roots. The formation of pyromorphite by Pband NHAP in soil was accordingly to interpret the dominant mechanism for Pb immobilization.