Effect Of Goethite On Stabilization Of Cadmium By Hydroxyapatite

With the acceleration of the process of industrialization and urbanization,soil pollution is grave in China.Among them,cadmium pollution is the most serious.The chemical passivation method based on phosphorus-based materials is one of the better methods for the treatment of farmland soil.However,the large amount of iron oxides contained in red soils in southern China can change the concentration,chemical form,and bioavailability of heavy metals and phosphorus in the soil,thus affecting the passivation of heavy metals by phosphorus-based materials.Therefore,based on the use of hydroxyapatite as a passivating agent,the effect of goethite on the content of phosphate and cadmium in the system was analyzed by adding different proportions of goethite to the solution system and soil system.At the same time,the effects of goethite on soil enzyme activities and forms of heavy metals were discussed through indoor incubation experiments,and hazard quotients were also evaluated to assess the potential effects of goethite or HAP-treated heavy metals contaminated soil on human health.The main results of the study are shown as follows:(1)The adsorption rate of phosphate in the solution system of goethite can reach87.5-100%in the goethite and HAP composite treatment with goethite and HAP mass ratio of 2:1 added.Under the conditions of system pH=4.5,5.5 and 6.5,the addition of goethite makes the solution pH increase 0.01-0.13,0.18-0.56 and 0.11-0.31 units,respectively,and reduced the phosphate by 6.88-13.3,4.70-15.7 and2.79-4.16mg/L,respectively.In a cadmium-containing solution system,the composite treatment with 2:1 mass ratio of goethite and HAP can increase the adsorption rate of phosphate by 67.6%more than that of 1:2 mass ratio.However,at pH=4.5,5.5 and 6.5,the p H,phosphate,calcium content and cadmium adsorption rate in the solution did not show significant differences.(2)In the soil system,the combination of goethite and HAP added in a 2:1 mass ratio can significantly increase the pH of the solution from 0.29 to 0.89 units compared to a single HAP treatment.In addition,in the treatment containing HAP,the addition of goethite can significantly reduce the desorption of phosphate from 0.15 to 0.38mg/L.Compared with the treatment without low molecular weight organic acid,the content of phosphate in the treatment of 10mmol/L citric acid and oxalic acid was increased by89.6 and 40.4mg/L,respectively.But compared to the single HAP treatment,the phosphate content in the composite treatment with 2:1 mass ratio was reduced by 29.3and 28.8mg/L,respectively.In a cadmium-containing solution system,the addition of goethite and HAP in a mass ratio of 2:1 increased the phosphate adsorption rate by32.8%compared to the composite treatment with a 1:2 mass ratio,which was also increased the pH and cadmium sorption of the solution by 0.97 units and 13.9%,respectively,compared to the single HAP treatment.However,under the conditions of acid rain pH=3.5,4.5 and 5.5,the calcium content and the cadmium adsorption rate in the solution showed no significant difference.(3)In indoor incubation experiments,a single 1%goethite or 1%HAP significantly increased soil pH by 0.73 and 0.61 units,respectively.At the same time,compared with the fixed rate of available soil heavy metals Cu,Cd and Pb in the control treatment,the addition of single 1%goethite increased by 41.6%,10.8%,and 50.2%,respectively,and the addition of a single 1%HAP increased by 98.3%,95.5%,and96.6%,respectively.Compared with the activities of soil catalase,urease and acid phosphatase in the control treatment,the addition of single 1%goethite increased by0.24,0.24,and 0.05mL/g,respectively,and the addition of a single 1%HAP increased0.65,1.05,and 0.11mL/g,respectively.However,in the complex treatment of goethite and HAP,the contents of available Cu,Cd and Pb and the activities of catalase,urease and acid phosphatase in the soil were not significantly affected by the single HAP treatment.Another,in the fractions of soil P,the composite treatment of 1%goethite and HAP significantly reduced the resin-P content of 51mg/kg compared to single HAP treatment,indicating that goethite can reduce the phosphorus release from phosphorus passivators reduces the risk of eutrophication.In addition,in the evaluation of human health risks,compared to the control treatment,the biocompatibility of 1%goethite and HAP on the heavy metals Cu,Cd,and Pb was reduced by 7.87%,15.3%,and 10.1%,respectively.Studies have shown that hazard quotients(Q_H)values for three heavy metals were less than the accepted risk limit in both adults and children,and did not pose health risks for people.However,it is worth noting that children’s Q_H values for Cu,Cu and Pb under different treatments are 10 times that of adults.Therefore,special attention must be paid to the harm of heavy metals to children’s health.