| Soil heavy metal pollution is one of the most conerned environmental issues at home and abroad.Once the heavy metal concentration in cropland soils?especially paddy soil?exceeds the safe limit,the health safety of human will be greatly threatened.Understanding the transportation and transformation characteristics of heavy metals in paddy soil is benefit for reasonably assessing and controlling environmental risk of heavy metals and providing important theoretical basis for contamianted soil remediation.To date,numerous of works have been conducted in the aspects of adsorption,transportation,speciation transformation and stabilization of heavy metals in paddy soil,however,these studies usually neglected the influence of real environment factors,such as soil spatial structure variation,effect of aging and moisture condition.Based on sufficiently summarizing previous research results and aiming at compensating the weakness of present studies,this research studied the adsorption,enrichment,migration and stabilization of heavy metals in soil by selecting red paddy soil as the study object and adopting the methods of batch adsorption,simulated pollution,soil column leaching and passivant stabilization experiments.This type of soil is widely distributed in south China and is severely polluted by heavy metals.Among them,considering from the angle of the heterogeneity of soil structure,we examined the adsorption characteristics of Cu,Zn and Cd onto different particle size aggregates and the influence of relevant factors include temperature,time and pH.In addition,the influence of different components(organic matter,Fe oxides?free and Amorphous Fe oxides?,Mn oxides was also evaluated by using selective components removal technology.Enrichment characteristics and fractions distribution of heavy meals in different aggregates along paddy soil profile was further studied.Incubation and soil column leaching experiments were conducted to investigate the effect of aging on the fractions transformation of heavy metals and the relationship between speciation and leachability was also analyzed.Finally,the influence of different moisture conditions?70%field capacity,flooding and wetting-drying cycle?on the stabilization effect under the conditions of adding some commonly used passivants?biochar,lime,Ca?H2PO4?2 and modified zeolite?into Cd polluted paddy soil was examined.A total of five interrelated research works were designed in this study and the obtained conclusions are as follow:1.Temperature,water/soil ratio,particle size,time and pH condition all could apparently influence the adsorption characteristics of heavy metals in paddy soil.The adsorption capacity increased with the increase of water/soil ratio,while with the rise of temperature,the variation trend of adsorption capacity of various heavy metals was different.There existed two stages?fast and slow?during the adsorption process of heavy metals in soil aggregates.Both the pseudo-first and pseudo-second models were successful to fit the adsorption process in the kinetic experiments and the adsorption rate of the<0.002 aggregate was the highest.The isothermal adsorption data of Cu and Zn could be well described with the Freundlich model,while Langmuir model was more suitable for fitting the data of Cd.Due to higher content in components such as organic matter,CEC and free Fe oxides,the<0.002 mm fraction aggregate was found to have the maximum adsorption capacity.The adsorption amount of metals increased as the increase of pH and the proportion of specific adsorption of the clay sized aggregate was higher than other sizes of aggregates.With the increase of pH,the desorption rate of Cu decreased,whereas the trend was opposite for Zn and Cd.2.The importance of various components was dfferent in influencing the adsorpiton beaviour of heavy metals in paddy soil.After removing different soil components,the adsorption behavior of Cd onto different particle size aggregates could still be well described by Langmuir and Freundlich models.Removing organic matter,amorphous Fe oxides and Mn oxides resulted in significantly decrease of Cd adsorption capacity,whereas an opposite result was observed for the treatment of removing free Fe oxides.The effect of removing unit mass of different soil components on the asorption capacity followed the order:Amorphous Fe oxides>Mn oxides>free Fe oxides>organic matter.Free and amorphous Fe oxides exhibited more remarkable influence on the desorption of Cd in paddy soil,while the influence of organic matter was much smaller.Furthermore,the desorption behavior of Cd in different particle size paddy soil aggregates was also different.3.The physicochemical properties of soil and enrichment of metals showed significant regularity in the vertical direction.The physicochemical properties of surface layer soil showed obvious difference along the vertical direction.In different soil layers?0-1,1-5,5-15 and 15-25 cm?,the pH,free Fe oxides content increased with the increase of soil depth,while the content of organic matter,CEC,electrical conductivity?EC?and microbial biomass C gradually increased.The variation trend of urease and catalase activity was contrary along the soil profile;catalase activity seemed to be easier to be influenced by heavy metal pollution.The concentration of heavy metal gradually decreased with the increase of soil depth.In different soil layers,the concentrations of Cu and Cd showed the highest values in the 2-0.25 mm fraction aggregate.For Zn and Pb,the highest values appeared in the<0.002 mm fraction aggregate,followed by the 2-0.25 mm fraction.The As concentration in the<0.002 mm fraction was also the highest,in the 0-1and 1-5 cm soil layers,the As concentrations in the 2-0.25 mm fractions were higher than that in the 0.25-0.05 and 0.05-0.002 mm,whereas the trend was opposite in the 5-15 and 15-25 cm soil layers.No significant regularity was observed in the distribution of Hg in different particle size soil aggregates.Except for Hg,the speciation of all the heavy metals was more stable in the deeper soil layers.Only Cu and Cd showed higher proportion of stable speciation in the<0.002 mm fraction aggregates.4.Aging effect could significantly influence the fractions distribution and migration ability of heavy metals in paddy soil,the content of exchangeable and HoAc soluble fraction in soil showed decreased trend with aging time.Power function was the most excellent to describe the variation of exchangeable fraction during the aging process,but unsuitable to describe the leaching characteristics of metals from soil columns.The leachability of Cu,Zn and Cd in paddy soil followed the order:Cd>Zn>Cu,the longer time the soil was contaminated,the less amounts of metals released from the soil.The reduction of exchangeable fraction caused by aging effect was the main reason for the decrease of metal mobility in paddy soil.5.Moisture condition posted obvious influence on the physicochemical properties of paddy soil and the immobilization effect of the passivators.Under the flooding condition,the pH value,free and amorphous Fe oxides,soluble organic matter and inorganic matter content apparently increased.The effect of different moisture condition on reducing the exchangeable fraction Cd concentration followed the order:flooding>wetting and drying recycle>70%field capacity,flooding condition has less influence on the variation of HoAc soluble Cd.On the whole,the stabilization function of biochar and lime was better than that of Ca?H2PO4?2 and modified zeolite,and flooding condition was benefit for the transformation of Cd form HoAc soluble fraction to more stable fractions.In summary,the adsorption,enrichment,transportation of heavy metals in paddy soil is a complicate environment process synthetically effected by the nature of metals,physicochemical properties of soil and ambient environment conditions.Influence factors existing in realistic environment should be considered when predicting environment risk of heavy metals and adopting restoration action.The research result obtained in this study will be helpful in understanding the environmental behavior of heavy metals,and providing certain guidance basis for reasonably predicting and evaluating the risk of heavy metals in realistic environment as well as providing important reference information for heavy metal polluted paddy soil restoration practice. |
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