Stabilization And Enhancement Of Heavy Metal Contaminated Soil

With the development of industry and agriculture,various heavy metals are always threatening human health due to the nonstandard and unscientific use of pesticides,fertilizers,livestock and poultry,as well as atmospheric sedimentation,sewage discharge/irrigation and other reasons.Heavy metals can accumulate continuously in the human body through the food chain,causing irreversible harm to the human body,and blocking the harm of heavy metals to the human body at the root and stabilizing the heavy metals in the soil has become an urgent problem to be solved.Taking paddy soil polluted by cadmium(Cd 31.64 mg/kg),lead(Pb 191.92 mg/kg)and zinc(Zn1368.36 mg/kg)around a lead and zinc mining area in Huludao as the research object,six stabilizing materials were prepared with biochar and iron hydroxide at different pH:(1)straw biochar(s),(2)iron-based biochar 2(st2),(3)iron-based biochar 7(st7),(4)iron-based biochar 12(st12),(5)iron hydroxide at pH 7(t7)(6)Iron hydroxide(t12)at pH 12 is used to stabilize and repair heavy metal contaminated soil.The adsorption effect of five stabilizing materials(s,st7,st12,t7,t12)0.25 g stirred at 300 r/min for 3 min,10 min,60 min,360 min,720 min,1440 min on single and multiple heavy metals(Cd,Pb,Zn)in the solution was studied through adsorption experiments.The concentration of single heavy metal solution was 50 mg/L,and the concentration of multiple mixed heavy metal solution was 18 mg/L for each heavy metal.The results showed that the stabilizing materials had good adsorption effect on Pb in a single solution,Moreover,t12 has the best adsorption effect.When the adsorption is balanced,the adsorption rate is 99.68%.The adsorption effect of various materials on Pb t12>st12>s>t7>st7.The maximum adsorption rates of materials st12 and t12 for Cd were 44.46% and 67.74%,respectively.Other materials had no obvious effect on Cd.The effect of materials t7 and st7 on Zn is not obvious.The maximum adsorption rates of materials s,st12 and t12 are 16.26%,41.08% and 46.44%respectively.In the mixed multi-component system,the five materials have the largest adsorption capacity for Pb,and the materials t12 and st12 have the largest adsorption capacity,both of which are more than 99%.The adsorption capacity of materials st7 and t7 for Cd and Zn in the multi-component system is not obvious compared with that in the single system,while the effects of materials s and st12 on Cd and Zn are obvious,and the effects of t12 are the best,and the maximum adsorption rates of Cd and Zn are86.55% and 84.55% respectively.Through the soil stabilization and remediation experiment of 6 stabilizing materials s,st2,st7,st12,t7 and t12 at 1%,3% and 5%,and sampling and analysis at 7,15 and 45 days after stabilization,the effects of 6 stabilizing materials on the speciation and distribution of 3 heavy metals,soil pH and EC were studied.The results showed that 6 stabilizing materials can stabilize Cd,Pb and Zn in soil within 45 days,The stabilizing effect rate of each material on Cd st12>st7>s>t12>t7>st2.The material st12 group can convert 5.43 mg/kg of the total content of weak acid extraction and reducible state into oxidizable state and residual state,which is the largest in each group.The stabilizing effect of six materials on Pb t12>st7>st12>t7>st2>s,material t12 can convert 52.40 mg/kg of the total content of weak acid extraction and reducible state to oxidizable state and residue state,and for Zn,the effect of six materials st12>st2>st7>s>t12>t7,material st12 can convert 114.72 mg/kg of Zn in the total content of weak acid extraction and reducible state to residue state and oxidizable state.The pH of soil CK group increased slightly under the condition of water supplement only,and the pH was 7.04 at 45 days.Materials s could increase the soil pH,while materials t7 and t12 showed a downward trend 30 days ago,and rose to the original soil pH level at 45 days.The EC of soil CK group increased slightly under the condition of water supplement only,and increased to 438 S/m at 45 days.All materials can increase the EC value in the soil,and at 45 days it is basically higher than that at 7 days,and the increase amount is positively related to the application amount.But within 45 days,due to the different application amount,the material reacts with the ions in the soil,so the change trend of different application amounts of the same material is different in 45 days,and the increase of material t7 is obvious.Through the freeze-thaw cycle experiment on the soil stabilized by six materials s,st2,st7,st12,t7 and t12 for 45 days,sampling and analysis were conducted at 3,7 and15 freeze-thaw times to study the effect of the stabilized materials on the distribution of three heavy metals,soil pH and EC during the freeze-thaw process.The results showed that the freeze-thaw cycle could activate Cd and Zn in the soil and stabilize Pb,The effect is more obvious with the increase of freezing and thawing times.Different stabilizing materials play a certain synergistic or inhibitory effect on the effect of freezing and thawing.For Cd in soil,material t12 has a significant inhibitory effect on the increase of weak acid extractable content caused by freezing and thawing,and the inhibitory effect is the strongest.Material t7 can increase the rate of the increase of weak acid extractable content in soil caused by freezing and thawing.Materials s,st12,t12,and st7 show different synergistic effects on the stability of freeze-thaw under different application rates.Material st7 has the strongest effect when the application rate is 1%,synergistically increasing the residual content to 65.60 mg/kg,when the application rate is 3%,st7 and st12 have the strongest effect,synergistically increasing the residual content to 68.75,68.99 mg/kg,when the application rate is 5%,the residual content of st12 group is 66.55 mg/kg,and materials s St2 can effectively inhibit the increase of zinc weak acid extraction content and the decrease of residual content in soil.After freeze-thaw for 3,7 and 15 times,the pH of CK group increased from 7.04 to 7.2,7.22 and 7.23.The pH of soil in each group basically showed an upward trend after 7 freeze-thaw cycles.Except for t12 group,the pH of each group basically showed a trend after 15 freeze-thaw cycles,and the change amount had little relationship with the applied amount.After freezing and thawing,the soil EC of CK group increased slightly.After 3,7 and 15 times of freezing and thawing,the soil EC was 455,458 and545 S/m,respectively.After freezing and thawing,the overall EC of each group showed a downward trend,and increased after 15 times of freezing and thawing.