MgO-montmorillonite Combined With Compost For Immobilization Of Heavy Metals In Contaminated Soils

Soil is the basis of human existence.Due to human’s greedy exploitation of natural resources and unreasonable discharge of industrial pollutants,the problem of soil heavy metal pollution cannot be ignored.It is reported that the over-limit rate of heavy metal in cultivated land of China has reached 19.4%,and the rate is even higher in some specific areas.The overall trend is not optimistic.According to China’s national soil pollution survey bulletin,the over-limit rate of heavy metals in cultivated land nationwide has reached 19.4 percent,and in some areas it is extremely higher.Heavy metals are a kind of metal elements that are very difficult to be biodegraded,and have the effect of bioamplification and biotoxicity,such as Pb,Cd,and Cr.Existing in the soil for a long time,they can accumulate and damage the soil environment,which will further endanger animals,plants,groundwater and even threaten human through the food chain.Therefore,the remediation of heavy metals in soil has become the focus of the world.Compost,due to its unique composition and physicochemical properties,has a good effect on remediation of soil heavy metal pollution,which is an efficient soil amendment for heavy metal.Compost is rich in organic materials,which can stabilize the heavy metals in the soil through adsorption,complexation,precipitation,redox and so on.Meanwhile,it leads to improve soil pore structure,soil fertility,and crop growth.However,over a long period of time,compost has a potential of decomposition to induce changes in soil pH,so heavy metals stabilized by compost may be reactivated and released.At present,there is still no good method to solve this problem.Clay minerals are naturally formed layered silicates on earth.They are cheap,non-toxic,high specific surface area,strong in ion exchange,and have good adsorption properties for heavy metals.Thus,they are commonly used to repair water bodies and soils polluted by heavy metals.Nevertheless,the effect of single clay on remediation for some heavy metal is not satisfactory.For example,single bauxite exhibits high performance for Pb and Cu immobilization in soil,while does inferior performance for Sb.It is expected to overcome the defects of single compost or single clay by combining compost and non-degradable inorganic clay minerals,in order to achieve long-term and highly efficient remediation effect on heavy metal contaminated soil.The main contents of this study include:(1)Pot experiment was conducted to evaluate the effects of different proportions of Mg O-montmorillonite(MMt)and compost on soil physical and chemical properties after 60 days remediation;(2)The heavy metals were extracted from the soil by TCLP and BCR methods,and the effects of different proportions of MMt combined compost on the mobility of heavy metals in the soil and their mechanism were analyzed;(3)Measure the accumulation of heavy metals in different parts of plants harvested from amended soil;(4)Analyze the effects of different proportions of MMt and compost on soil microbial environment.In this study,different proportions of MMt and compost as combined agents were used to remediation of single cadmium-contaminated soil(soil 1)and multi-heavymetal(Cu,Cd,Pb,Zn)contaminated soil(soil 2),and pot experiment was conducted to explore their remediation effect.700 g test soil was added into each pot,and then 3%MMt and compost mixture(ratios set as 1:9,2:8,4:6)was added,next all the materials were thoroughly mixed.Water spinaches were planted in each pot for 60 days.Vegetables and soils were collected to determine their physical and chemical properties.Both for soil 1 and soil 2,alone addition of compost decreased soil pH and MMt increased soil pH.Hence,the MMt-compost combinations could reduce the negative effects of pH decreasing caused by compost,thus enhancing the immobilization of metals in soil.Compared with control treatments,the MMt-compost combinations increased the water extract organic carbon(WEOC)by 61.9～86.4% in soil 1,and increased the WEOC by 31.6～184.9% in soil 2.The optimized MMt-compost at ratio of 1:9 decreased TCLP-extractable Cd in soil 1 by 35.2% and declined the F1 fraction of Cd in BCR procedure by 14.7%,which exhibited better performance than each separately addition.For soil 2,the ratio of 2:8 between MMt and compost showed the best efficiency,the TCLP-extractable Cu,Cd,Pb and Zn decreased 43.4%,26.9%,38.9%and 9.6%,respectively.The F1 fraction of Cu,Cd and Pb in BCR decreased 27.7%,9.3% and 29.9%,while Zn almost was not changed.Both in soil 1 and soil 2,MMtcompost combinations significantly decreased heavy metals accumulation of water spinach in heavy-metal-contaminated soils.Compared with control treatment,the Cd content of water spinaches from soil 1 was significantly decreased by 18.3～49.9% for roots,17.7～58.3% for stems,7.1～56.2% for leaves.For amended soil 2,with different proportion of MMt and compost,the Cu content in roots,stems and leaves of water spinaches decreased by 67.8～98.9%,3.2～80.9% and 56.4～90.9%,respectively;the Pb content of the corresponding parts decreased by 80.2～93.8%,80.3～97.4% and57.7～93.1%;the Zn content decreased by 75.3～90.8%,56.7～94.2%,and 62.8～95.2%;and the Cd content decreased by 47.8～80.7%,42.9～83.4%,and 61.2～92.5%.For soil 1,different proportions of amendments significantly enhanced the activity of soil urease,and the M2C8.S treatment showed the highest dehydrogenase activity within soil,which was increased by 1.95 times compared with the control.Similarly,urease activity in soil 2 was significantly boosted.For the M1C9.M,M2C8.M and M4C6.M treatment groups,their urease activities increased by more than 1.8 times.The urease activity in M2C8.M soil was twice that in M10C0.M soil and 10 times that in M0C10.M soil.The dehydrogenase activity in M2C8.M soil was the highest,which was increased by 2.02 times compared with the control.In addition,the microbial diversity in amended soil 1 and soil 2 was both boosted dramatically.There may be two reasons for the enhancement of urease,dehydrogenase and microbial diversity in amended soils.On the one hand,the addition of MMt combined with compost reduces the migration of heavy metals in the soil,so the toxic effect of heavy metals on microorganisms is suppressed.On the other hand,compost is rich in nutrients,which could accelerate the metabolism,growth and reproduction of microorganisms in soil to boost the microbial diversity.Redundancy analysis(RDA)results suggested that heavy metals content and pH are the two main factors affecting microbial community.Both for soil 1 and soil 2,MMt combined with compost showed better effect than the MMt or compost alone for heavy metal remediation,showing a mutually reinforcing effect among them.Both MMt and compost can effectively reduce the mobilization of heavy metals in soils,reduce the accumulation of heavy metals by plants and improve the soil environment.In the meantime,they are widely available,inexpensive and non-toxic.Therefore,MMt combined with compost is effective and feasible combining amendment for ameliorating heavy metals in soils.