Stabilization And Effect Evaluation Of Hexavalent Chromium In Soil By Pine Bark And Its Modified Materials

Hexavalent chromium [Cr(Ⅵ)] has genotoxicity and carcinogenicity characteristics,and its environmental risks are of great concern.Pine bark is a common forestry waste,which is simple and easy to obtain,low in cost,and will not produce secondary pollution while applied in the environemtal remediation.Pine bark is rich in reducing substances such as proanthocyanidins,which can be used to reduce Cr(Ⅵ)into Cr(III).In this study,experiments including pine bark modification,screening of optimal modified material in aqueous system,and soil incubation were conducted to explore the reduction/sorption capacities of pine bark and its modified materials and the stabilization effects of Cr(Ⅵ)in soil,followed by leaching toxicity assessment of the remediated soil using TCLP(Toxicity Characteristic Leaching Procedure)method.The main contents and results are as follows:1)Three kinds of modified materials(PA-PB、DTAB-PB、CTAB-PB)were obtained by modifying pine bark with phosphoric acid and two cationic surfactants with different chain lengths,namely dodecyltrimethylammonium bromide(DTAB)and cetyltrimethylammonium bromide(CTAB).The removal effects and mechanisms of Cr(Ⅵ)by the original pine bark(Pinus sylvestris var.mongolica Litv.)and its modified materials were investigated by batch experiments in aqueous phase.The results showed that all the three modification methods enhanced the removal of Cr(Ⅵ),among which CTAB-PB showed the highest removal efficiency and adsorption capacity of Cr(Ⅵ),achieving 98.2% removal in 10 minutes of reaction,and it also has good removal effect at high p H.The maximum adsorption capacities of CTAB-PB,PA-PB,DTAB-PB,and PB were 88.1 mg/g,80.5 mg/g,74.5 mg/g,and 73.5 mg/g,respectively.FTIR analysis showed that DTAB-PB,CTAB-PB could increase the electrostatic adsorption and the formation of complex.The XPS analysis proved that there is more Cr(Ⅵ)than Cr(III)on the surface of PB and PA-PB after reaction,whereas Cr(III)is the dominate species on the surface of CTAB-PB and DTAB-PB,indicating that the modification with CTAB and DTAB increased the reduction of Cr(Ⅵ)to Cr(III).The removal of Cr(Ⅵ)from solution can be well described by the Freundlich isotherm adsorption model,and the reaction process follows a pseudo second order kinetic equation.(2)Jiangxi Red Soil spiked with Cr(Ⅵ)was used as the experimental soil,which was stabilizated with PB and CTAB-PB at 2%,5%,10%,15%,respectively.The experimental results showed that the reduction of Cr(Ⅵ)by PB and CTAB-PB generally increased with increasing the material additions,and the CTAB-PB treatment exhibited better reduction effects and reduction rates than the corresponding PB treatments.At the initial stage of stabilization,the reaction rate is rapid,and the equilibrium was reached after 15 days for the treatments with 10% and 15% addition of PB and CTAB.The Cr(Ⅵ)removal was the highest at 30 d with more than 93%removal for all the treatments of both materials.During the stabilization process,soil Cr(Ⅵ)concentration decreased gradually,while the concentration of soil Cr(III)increased continuously,indicating that PB and CTAB-PB remediated Cr(Ⅵ)contaminated soil by reducing Cr(Ⅵ)to Cr(III).3)The leaching toxicity of Cr(Ⅵ)in soil treated with PB and CTAB-PB was studied by TCLP method.The results showed that both PB and CTAB-PB could significantly reduce the leaching concentration of soil Cr(Ⅵ)in a short time,and the decreasing effect increased with increasing PB and CTAB-PB addition.Except for 15%PB treatment,significantly lower concentration of Cr(Ⅵ)in the leaching solution was observed on 0.5 d or 1 d.CTAB-PB and PB can not only reduce the concentration of soil Cr(Ⅵ)in a short time,but also maintain a low leaching level in the later stage.