| With the rapid development of mining and industrial production, the soil contamination by heavy metals is more and more serious. Heavy metals are not only cause the crop yield reduction but also endanger human body health by bioaccumulation. The "Production and Repair" method of chemical fixation to remedy heavy metals contaminated soil is very popular in China. But, the immobilizated heavy metals may be activated under the effect of acid rain and the apply of phosphorus whether cause eutrophication of water bodies is not clear. So we studied the release characteristics of phosphorus and cadmium from a contaminated soil under simulated acid rain with pH of 3.5,4.5 and 5.6 in the paper through a leaching experiment for the untreated soil and hydroxyapatite amended soil with a rate of 0.5%(w/w). The results showed that:In the leaching solution, the pH was decreased with the decreasing pH of simulated acid rain. However, the pH of leaching solution increased 0.3-1.5 units in hydroxyapatite treated soil compared with the untreated soil because of the dissolving hydroxyapatite. Moreover, the contents of total phosphorus and orthophosphate in the leaching solution were both increased with the decreasing pH of simulated acid rain. With the dissolution of hydroxyapatite, the total concentrations of phosphorus in 1-3 L and 4-6 Lwere 2.70-3.55 and 1.25-2.15 times higher than the class V guideline (0.4 mg·L-1) of the Chinese National Quality Standards for Surface Waters (GB 3838-2002) respectively. The concentrations of Cd in leaching solution decreased significantly in hydroxyapatite amended soil compared with the untreated soil. The contents of Cd in the leaching solution of the untreated soil were increased with the decreasing pH of simulated acid rain, but they decreased gradually in hydroxyapatite amended soil because of the fixation of phosphorus. Our results indicated that the application of hydroxyapatite could decrease effectively the contents of Cd in leaching solution, but had a potential risk for water eutrophication due to the release of phosphorus.The pH of control soil was decreased with the leaching of simulated acid rain, but the HAP treated soil increased the acid buffer capacity of soil and increased about 1 unit of pH compared with the control soil. The decreased pH of simulated acid rain increased the migration of Cd to the low depth of soil column in the control soil. But the Cd was strongly fixed in HAP treated soil and concentrations of Cd were 8.54-10.0 and 3.13-3.29 mg-kg"1 higher than those of the control soil in the depths of 0-5 and 5-10 cm, respectively. Under the leaching of simulated acid rain, the concentrations of TCLP-extractable Cd in the control soil were increased from 0.17-0.23 mg·L-1 at the depth of 0-5 cm to 0.61-0.68 mg·L-1 at the depth of 10-20 cm, but they ranged from 0.45 to 0.50 mg·L-1 at the depth of 0-20 cm in the HAP treated soil. Overall, the HAP treated soil significantly increased the number of fungi, actinomycetes and the activity of urease, acid phosphatase compared with the control soil. The bioaccessible concentration and bioaccessibility of Cd in gastric and small intestinal phases in all treated soils decreased with the decreasing pH of simulated acid rain. Moreover, the bioaccessible concentration and bioaccessibility of Cd in gastric and small intestinal phases in the HAP treated soil were higher than those in the control soil, but bioaccessibility of Cd in small intestinal phase was lower. In gastric and small intestinal phases, the contribution rates of the oral ingestion soil Cd to the provisional tolerable monthly intake (PTMI) that recommended by WHO in all soils were decreased with the decreasing pH of simulated acid rain, but they were higher in the HAP treated soil than those in the control soil. The findings of this study indicated that HAP treated soil can effectively increase the acid buffer capacity and decrease the leaching loss of Cd. But bioaccessibility of Cd and health risk from the oral ingestion of soil Cd in HAP treated soil are higher than the control soil, which need to pay more attentions when HAP is applied to the immobilization of Cd-contaminated acidic soil. |
PDF Full Text Request