Studies On Mechanism And Remediation Of Nano-Magnesium Hydroxide In Cd Contaminated Soils

With the rapid development of industry and agriculture,the continuous development of mines,the wanton discharge of cadmium-containing wastewater and the abuse of pesticides and fertilizers,the cadmium pollution problem has spread all over the world.Soil cadmium pollution in China has become an environmental problem that can not be ignored.It is urgent to seek economic and effective remediation technology without secondary pollution.In-situ chemical fixation remediation technology is one of the important ways to control heavy metal pollution in soil.By applying passivators to soil,the forms of heavy metals in soil can be regulated and changed,the exchangeable components and their migration in soil can be reduced,thus the bioavailability of cadmium can be reduced.Therefore,finding environmentally friendly and efficient heavy metal passivators has become the focus of research.Nano-magnesium hydroxide is a new inorganic material with small particle size,large specific surface area,non-toxic,harmless and strong adsorption capacity.It has been widely used in the treatment of heavy metals in wastewater,and is called green and safe water treatment agent.Although nano-magnesium hydroxide has been reported in the field of environmental protection and fertilizer,there are few reports on the application of nano-magnesium hydroxide in the remediation of heavy metal contaminated soil at home and abroad.The relationship between the remediation mechanism,remediation effect and the effect on plant growth of nano-magnesium hydroxide on cadmium contaminated soil is also unclear.Therefore,nano-magnesium hydroxide（nMg）and ordinary magnesium hydroxide（oMg）were used as cadmium-contaminated soil passivators in this experiment.The effects and mechanisms of nano-magnesium hydroxide and ordinary magnesium hydroxide on the remediation of cadmium-contaminated soil were investigated,and the appropriate amount of nano-magnesium hydroxide for the remediation of cadmium-contaminated soil and the safe production of vegetables was determined.The main results are as follows:1.The effects of nano-magnesium hydroxide and ordinary magnesium hydroxide on the distribution of cadmium forms in neutral soils（FDC）under different levels of cadmium contamination（1,5,10 and 15 mg·kg-¹）,with different dosages（100,200 and 300 mg·kg-¹）and with incubation time（0,1,4,7,14,21 and 28 days）and on soil pH were studied.1)At the end of 28 days,with the increase of cadmium concentration,soil EX-Cd FDC increased gradually,and soil EX-Cd FDC increased slightly under different cadmium concentration,which did not reach dynamic equilibrium.Finally,soil EX-Cd FDC was lower than that before culture.At the end of 3 weeks’cadmium aging,that is,on the 0th day of incubation,EX-Cd existed in all cadmium concentrations.At 1,5,10 and 15 mg·kg-¹ cadmium concentrations,EX-Cd FDC was66.7%,73.7%,77.4%and 81.8%respectively,significantly higher than other forms（P<0.05）,and all of them were EX-Cd>CAB-Cd>RES-Cd>Mn-Cd>FeOM-Cd.On the fourth day of incubation,soil EX-Cd FDC decreased sharply,while soil CAB-Cd,FeMn-Cd and RES-Cd FDC increased.Compared with the first day,soil EX-Cd FDC decreased by 54.6%-73.1%,12.1%-30.4%,35.0%-51.7%and 23.1%-36.4%at 1,5,10 and 15 mg·kg-¹ cadmium concentrations,respectively.On the 14th day,the EX-Cd FDC of soil reached the lowest level at 1 and 5 mg·kg-¹ cadmium concentration.The EX-Cd FDC of soil at 1 mg·kg-¹ cadmium concentration was only 5.7%-17.6%.The EX-Cd FDC of soil at 10 and 15 mg·kg-¹ cadmium concentration increased slightly,but began to stabilize.2)At the end of incubation,the addition of nano-magnesium hydroxide and ordinary magnesium hydroxide reduced the EX-Cd FDC of soil at various cadmium concentrations.With the increase of the amount of nano-magnesium hydroxide,the EX-Cd FDC of soil tended to decrease.Under the same amount of nano-magnesium hydroxide,the effect of reducing EX-Cd FDC of soil was better than that of ordinary magnesium hydroxide.Magnesium hydroxide addition also reduced soil CAB-Cd FDC at 1 and 5 mg·kg-¹ cadmium concentration,increased soil CAB-Cd FDC at 10and 15 mg·kg-¹ cadmium concentration,and increased soil FeMn-Cd,OM-Cd and RES-Cd FDC at different cadmium concentration.In the 0-28 days of incubation,the EX-Cd FDC of soils treated with nano-magnesium hydroxide and ordinary magnesium hydroxide decreased by 11.4%-67.7%,7.8%-37.2%,7.7%-36.4%,5.0%-28.8%（nano-magnesium hydroxide）and 0.5%-49.5%,0.6%-15.0%,1.0%-18.1%,0.7%-14.6%（ordinary magnesium hydroxide）compared with CK,respectively.3)The addition of nano-magnesium hydroxide and ordinary magnesium hydroxide increased soil pH,and increased with the increase of the amount of nano-magnesium hydroxide.Soil pH was negatively correlated with soil EX-Cd content（P<0.01）,and positively correlated with soil FeMn-Cd,OM-Cd and RES-Cd content.There was a significant negative correlation between soil pH and soil CAB-Cd content at 1 and 5 mg·kg-¹ cadmium concentrations（P<0.05）,and a significant positive correlation at 10 and 15 mg·kg-¹ cadmium concentrations（P<0.01）.2.The changes of exchangeable cadmium content in different types of soils（acidic and alkaline soils）with different levels of cadmium contamination（1,5,10 and 15 mg·kg-¹）,different amounts of nano-magnesium hydroxide（100,200 and 300 mg·kg-¹）and ordinary magnesium hydroxide（100,200 and 300 mg·kg-¹）as well as effect of soil pH were studied by continuous incubation in laboratory.1)During the whole cultivation process,the EX-Cd content of alkaline soil was lower than that of acidic soil.The addition of nano-magnesium hydroxide and ordinary magnesium hydroxide reduced the EX-Cd content of acidic soil and alkaline soil at various cadmium concentrations.During 0-28 days of incubation,EX-Cd content in acidic soils treated with nano-magnesium hydroxide and ordinary magnesium hydroxide at 1,5,10 and 15 mg·kg-¹ cadmium concentration compare of CK was decreased 0.7%-35.7%,3.9%-29.2%,3.5%-26.9%,5.6%-23.1%（nano magnesium hydroxide）and 5.7%-35.8%,2.7%-24.5%,2.7%-21.5%（oMg-L treatment on the 14st day increased 5.4%）,0.4%-23.1%（ordinary magnesium hydroxide）;alkaline soil EX-Cd content decreased by 4.6%⁶3.8%,4.2%-26.1%,0.7%-38.0%,4.8%-27.8%（nano-magnesium hydroxide）and 2.0%-54.0%,2.5%-15.8%（oMg-L treatment on the 7th day increased 2.5%）,1.1%-17.5%（oMg-L and oMg-M treatment on the 21st day increased 0.6%and 1.8%,oMg-L treatment on the28st day increased 2.6%）,1.8%-23.0%（ordinary magnesium hydroxide）.At the end of incubation,the EX-Cd content of acidic soil and alkaline soil decreased with the increase of nano-magnesium hydroxide and ordinary magnesium hydroxide.Under the same amount of nano-magnesium hydroxide,the effect of reducing EX-Cd content of soil was better than that of ordinary magnesium hydroxide.2)Soil pH is an important factor affecting the availability of cadmium in soil.There was a linear negative correlation between pH and EX-Cd content in acidic and alkaline soils.The addition of nano-magnesium hydroxide and ordinary magnesium hydroxide increased the pH of acidic and alkaline soils,and increased with the increase of the amount of nano-magnesium hydroxide.3.Soil culture experiments were conducted to study the effects of nano-magnesium hydroxide and ordinary magnesium hydroxide on the distribution of Cd forms,soil pH,vegetable growth,photosynthetic characteristics,antioxidant enzyme activity,Cd content and Cd accumulation under different levels of cadmium pollution（1 and 5 mg·kg-¹）.The effects of different amounts of nano-magnesium hydroxide and ordinary magnesium hydroxide（0,100,200 and 300 mg·kg-¹）were studied.The correlation among soil pH,soil Cd content and Cd content in vegetable parts was further analyzed.To clarify the mechanism of remediation of cadmium contaminated soil by nano-magnesium hydroxide.1)Both nano-magnesium hydroxide and ordinary magnesium hydroxide can alleviate the toxicity of cadmium to chinese cabbage and promote the growth of chinese cabbage.At the same time,high amount of nano-particles can produce certain toxic effects on crops and inhibit the growth of crops.The growth of chinese cabbage was better under the condition of exogenous low cadmium（1 mg·kg-¹）,low amount（100 mg·kg-¹）of nano-magnesium hydroxide,and the growth of chinese cabbage was further promoted under the condition of exogenous high cadmium（5 mg·kg-¹）.The biomass of leaves and petioles treated with high amount（300 mg·kg-¹）of nano-magnesium hydroxide was higher.The leaf and petiole biomass of two chinese cabbage varieties,Liangqing and Chunxiawang,increased with the increase of magnesium hydroxide application.2)Cadmium stress affected the photosynthesis of chinese cabbage,while magnesium hydroxide enhanced the photosynthesis of chinese cabbage leaves,alleviating the damage to plant growth under cadmium stress.High cadmium treatment（5 mg·kg-¹）reduced Pn,Gs and Ci in leaves of Chunxiawang varieties,and increased Pn,Gs and Tr in leaves of Chunxiawang varieties after applying nano-magnesium hydroxide and ordinary magnesium hydroxide.Pn,Gs,Tr and Ci increased with the increase of application amount.3)Nano-magnesium hydroxide and ordinary magnesium hydroxide can reduce the cadmium content in different parts of chinese cabbage of Liangqing and Chunxiawang varieties,and with the increase of dosage,the effect of nano-magnesium hydroxide on reducing cadmium content in different parts of chinese cabbage is better than that of ordinary magnesium hydroxide.Compared with non-magnesium hydroxide treatment,nano-Mg（OH）₂ and ordinary Mg（OH）₂ reduced cadmium content in different parts of two varieties of chinese cabbage by 0.1%-30.3%and 1.6%-40.8%respectively.Liangqing chinese cabbage has stronger cadmium accumulation ability in soil.The cadmium accumulation in leaves and petioles of Liangqing variety is higher than that of Chunxiawang variety in both high and low cadmium conditions.4)Cadmium in the aerial parts of Liangqing and Chunxiawang varieties were mainly in the form of sodium chloride extraction,followed by residue and deionization,and the contents of acetic acid and hydrochloric acid were the least.The ethanol extract,deionized water extract,sodium chloride extract and total cadmium extract of two varieties of chinese cabbage were effectively reduced by applying nano-magnesium hydroxide and ordinary magnesium hydroxide.5)Exchangeable cadmium content in soil is an important factor affecting cadmium uptake by plants.Cadmium content in different parts of two varieties of chinese cabbage was significantly or very significantly positively correlated with exchangeable cadmium content in soil,and negatively correlated with CAB-Cd,FeMn-Cd,OM-Cd and RES-Cd content in soil.The application of nano-magnesium hydroxide and ordinary magnesium hydroxide increased the soil pH of two varieties of chinese cabbage（Liangqing and Chunxiawang）,decreased the exchangeable cadmium content in soil,and reduced the cadmium content in different parts of chinese cabbage.4.Field experiments were conducted to study the effects of nano-magnesium hydroxide and ordinary magnesium hydroxide（0,2.25,4.50 and 6.75 kg·hm-²）on vegetable yield,Cd uptake,soil available Cd content and total Cd content in Cd-contaminated vegetable garden soils.To verify the effect and feasibility of nano-magnesium hydroxide in practical application,and to determine the optimal application amount of nano-magnesium hydroxide in the remediation and treatment of cadmium contaminated soil in the field.The results showed that the application of nano-magnesium hydroxide and ordinary magnesium hydroxide reduced the content of available cadmium in soil（28.0%-33.4%and 18.4%-26.3%）and the content of cadmium in different parts of chinese cabbage（6.3%-31.6%and 1.1%-20.9%）.It was conducive to the safe production of vegetables,and the best reduction effect was high amount of nano-magnesium hydroxide（6.75 kg·hm-²）.The yield of chinese cabbage was also increased by nano-magnesium hydroxide and ordinary magnesium hydroxide（6.5%-21.0%and 4.1%-22.3%）.The yield of ordinary magnesium hydroxide was the highest with medium application rate（4.50 kg·hm-²）,112.542 kg·cell-¹.The yield of nano-magnesium hydroxide was the best with low application rate（2.25 kg·hm-²）,and the yield was111.308 kg·cell-¹.In summary,compared with ordinary magnesium hydroxide,nano-magnesium hydroxide has better effect on the safe production of cadmium-contaminated soil and crops because of its small particle size,large specific surface area,strong adsorptive capacity for cadmium and improving soil pH.It is more suitable for the restoration of cadmium-contaminated soil,but the appropriate amount of application should be mastered in order to avoid yield reduction.