Remediation Effects And Safety Evaluation Of Red Mud Based Passivator On Cd-Contaminated Paddy Field

The area of Cd-polluted cultivated land is very wide in China.At present,the Cd pollution situation in acid paddy soil in southern China is serious,and Cd in soil and rice is more than standard,it is necessary to remediate the Cd-polluted farmland to ensure the food security.At present,passivation which does not affect the normal farming is popular for its simple operation and efficient Cd reduction and.But it needs continous appllication to maintain the passivation effect,so the safety evaluation of passivator in farmland remediation application has become inevitable.Existing researches show that the red mud and it mixed with other materials can effectively reduce the content of the available Cd in soil and Cd in plant,howevert as an industrial by-product,the impact of the soil ecosystem also needs a more comprehensive and systematic study and evaluation.In this research,red mud,diatomite and lime as raw materials were composited to a passivator which was red mud based passivator,and it investigated the passivation effect of Cd,mechanism of action and duration,effects on soil environment and microbial community by soil culture experiment,pot experiment,plot test and field application.At the same time,the environment safety in agriculture was evaluated comprehensively.The main conclusions of this paper were shown as follows:The passivation effect of red mud based materials of different proportions was studied by the index of the available Cd content in soil.The results showed that the available Cd content in soil significantly decreased after 56 days,the range of descent was from 19.65%to 54.89%,while there was an increase in pH of 0.97¹.31 units.According to the passivation results of this test,the optimum compound ratio was:red mud:diatomite:calcium oxide=5:3:2.The adsorption experiment was conducted to study the adsorption mechanism of Cd by red mud based passivator.The result of comparison of the passivator before and after adsorption showed that:the red mud passivator had a high specific surface area,-OH,Si-O,Fe-O and other groups involved in the adsorption process of Cd ²⁺,but the structure of passivator was not damaged,the adsorption of Cd ²⁺is a process of physical and chemical adsorption.A pot experiment was conducted to study the reduction of Cd content in rice and the influence of the rice growth by adding the amount of red mud based passivator,and the results showed that the passivator could significantly reduce the Cd content in brown rice of acidic moderate Cd-contaminated soil,and with the increasing of adding amounts of the passivator,Cd in brown rice decreased and the declining range was from 46.44%to84.98%.In contrast,there was no obvious effect in alkaline severe Cd-contaminated soil.At the same time,the addition of passivator in a certain amount had a certain effect on the plant height and leaf area,but little in chlorophyll content.Also there were small impacts on physiological indexes and rice yield in acidic moderately Cd-contaminated soil and a larger impact in alkaline severely Cd-contaminated soil.Compared with the blank control,acidic moderate Cd-contaminated soils with addition of passivator at 0.1⁰.3%（w/w）,had no significant effect on plant height and chlorophyll,one hundred grains weight,spike,seed setting rate and yield;and had no significant effect on the plant height,leaf area and chlorophyll,but other yield elements except seed setting rate decreased significantly in most treatments in alkaline basic severely Cd-contaminated soil with addition of passivator at 0.2%⁰.6%（w/w）.Besides,the addition of passivator could improve the pH value and reduce the available Cd content in acidic moderately Cd-contaminated soil.The more addition of passivator the pH value increased more.There were significant differences between the control and the treatments of addition at 0.5%（w/w）and above in pH value and the available Cd content in soil,however there was no obvious regularity in alkaline severely Cd-contaminated soil.Combined with the results of the effects on rice growth,Cd content in brown rice,and pH value,the available Cd content in soil,the red mud based passivator had a significant reduction of Cd content in brown rice in Changsha country soil and had no obvious effect on the growth of rice,and the recommended application rate of red mud based passivator was 0.1%-0.3%（w/w）.After leaching of Cd-contaminated soil by simulated acid rain for 9 weeks,the following conclusions were drawn:the leaching solution of alkaline severely Cd-contaminated soil was alkaline,and the leaching solution of acidic moderately Cd-contaminated soils was acidic.Compared with the control,addition of passivator could improve the pH value in solution of acid soil,but it had little effect on the leaching solution of alkaline soil.There was no leaching risk in acidic moderately Cd-contaminated soil,however Cd could leach from the alkaline severely Cd-contaminated soil under the leaching of simulated acid rain.Addition of red mud based passivator could reduce Cd concentration in the leaching solution greatly.A yearlong plot test in Changsha County（both early and late rice）was conducted to investigate the effect of red mud based passivator on remediation of acidic Cd-polluted paddy field and the influences on soil environment and microbial community.The results showed that,there was no effect on leaf area,plant height and chlorophyll content in the process of rice growth when applying passivator at 0.3-0.6 kg/m² in early rice or both in two continuous seasons,which was in agreement with the results of the pot experiment.Also unlike the pot experiment,compared with the control,seed setting rate and yield of rice increased in two seasons.Compared with the control,adding red mud passivator in soil could reduce Cd content in plants and grains significantly,and these effects had certain continuity,the greater the added amount,the better the reduction and the continuity.The Cd reduction in the treatments of two seasons by passivator was better than that of the treatments of only in early rice.The Cd reduction rate of brown rice in early rice was48.14%and 66.15%,and the late rice was 19.92%,49%,47.01%and 72.11%respectively.The Cd content in different parts of rice plants was:root>stem>leaf>rice husk>brown rice,and the late rice was:root>stem>leaf>brown rice>rice husk.Compared with the control,there was a certain decrease in bioconcentration factors of plant and little influence on the transfer of Cd from root to other parts of rice.In the plot test,adding red mud based passivator could improve pH value of soil significantly,the greater the amount of addition,the greater the pH value increased,and the pH changed slowly with the growth of rice,which also had continuity in the second season.Compared with the control,these treatments of application of red mud based passivator did not cause significant losses in nutrient elements of N,P and K,also did not have a significant impact on the organic matter,cation exchange capacity.These treatments could reduce EXC-Cd content in soil significantly,improve the low bioavailability of other forms of Cd in two seasons,and the content of CA-Cd increased mainly in early rice and the content of Fe-Mn-Cd increased mainly in late rice.The more the amount applied,the greater the decline of EXC-Cd content,and the continuous application of passivator in two seasons was higher than that of only in early rice.The correlation analysis showed that Cd content in early rice was negatively correlated with CA-Cd content.Brown rice Cd in late rice was positively correlated with EXC-Cd content in soil,and this indicated that red mud passivator could reduce Cd content in brown rice by changing the form of Cd in soil.In the plot test,the number of soil bacteria and fungi increased and actinomycetes did not changed,The urease and catalase activities of soil increased in a certain extent,and soil microbial biomass carbon content increased by adding red mud passivator in soil significantly.Also the application of passivator could increase microbial biomass carbon content in rice field continuously.After adding red mud passivator,there was no significant difference in OTU richness of soil,Shannon-Wiener,relative abundance of main groups and OTUs relative abundance of 98.35%.In addition to the application of passivator at 0.3 kg/m² in the mature stage of late rice,the composition of soil bacterial community changed significantly,along with the bacterial community composition in control soil samples in other periods not different from treatments samples.In general,adding red mud based passivator had no significant influence on soil bacterial diversity,community composition and relative abundance.The remediation of paddy fields of 100 acres showed that:the application of red mud based passivator in field could reduce the Cd content of rice effectively,and make the Cd content of brown rice reach to the national food safety standard limit.It was an easy operation,and did not affect the normal cultivation,while the yield of rice increased slightly.Meanwhile the application of red mud based passivator could improve soil pH value relieve the soil acidification,reduce the content of available Cd in soil,and simultaneously it had no obvious effect on organic matter of soil.It could be seen from this research that the red mud based passivator had a good effect on passivation of Cd-poluuted paddy soil,which could reduce the leaching of Cd under the acid rain and slow down the soil acidification.Under a certain amount of passivator application and the continuous application in two seasons,it could significantly reduce Cd content in brown rice,not affect rice growth,and increase yield of rice slightly.At the same time,those elements were systematically analyzed that soil cation exchange capacity,organic matter content,available nitrogen,phosphorus and potassium,microbial number,soil enzyme activity and microbial community.The results showed that red mud based passivator had no negative effect on these environmental indicators.The field application further provided scientific guidance for the spread of red mud based passivator in a larger area.