Transformation And Bioavailability Of Arsenic In Soil

Arsenic contamination in soil has become a concern worldwide. The studies ofarsenic forms/species and bioavailability are the foundation for pollution recognition, riskassessment and remediation criteria setting. Based on the reviews of soil arseniccontamination, a rhizobag pot experiment and a pot trial were conducted to investigatestudy the adsorption-desorption characteristics, speciation, biovolatilization, Absorption,accumulation and distribution of arsenic and agrobiology of winter wheat or rape. Themain results are as follows.1. A batch method was adopted to study the adsorption-desorption characteristics ofaresnic in rhizosphere and non-rhizosphere soil of wheat or rape, using a rhizobag potexperiment. The results showed that along with concentration of aresnic increasing,adsorption amounts increase, and adsorption rates decrease in rhizosphere andnon-rhizosphere soil of wheat or rape. The adsorption amount of As⁵⁺ in soil was morethan that of As³⁺. the adsorption amount of arsenic in rhizosphere was more than that ofnon-rhizosphere. But at low concentration of arsenic (80 mg·kg^-1), the difference betweenadsorption amount of arsenic was not remarkable in both rhizosphere and non-rhizosphere.The desorption amount of arsenic increased with arsenic amount adsorbed and desorptionrates decrease in rhizosphere and non-rhizosphere soil of wheat or rape, the desorptionamount of As³⁺ in soil was more than that of As⁵⁺.2. A sequential extraction technique was adopted to study the speciation of arsenic inrhizosphere by a rhizobag pot experiment. The results showed that most of arsenic existedin the residual species at the slight pollution of arsenic. The exchangeable arsenic andcarbonate-bound arsenic became the predominant species, and the residual arsenicsignificantly decreased at the higher level of arsenic. These results demonstrated that theresidual species of arsenic transformed to exchangeable and carbonate-bound arsenicspecies in rhizosphere with the increase in arsenic pollution. Therefore, the phytotoxicityof arsenic was stronger in rhizosphere.3. A study was conducted by pot experiment to analyses the effect of different arsenicapplication on absorption, accumulation and distribution of arsenic in winter wheat orrape. The result showed that the arsenic applied in soil increased bioaccumulationfactor(BCF) of arsenic in crop, and resulted in accumulation of shoots, roots,. And thecontent of arsenic in the plant in creased with that applied in the soil. However, thearsenic applied in the soil could decrease the translocation factor (TF), thatis, the plant'sability to transfer arsenic from roots to shoots, Arsenic could be distributed in every organof the plant. The regular distribution pattern of arsenic in winter wheat or rape wasroots＞shoots. The absorption of arsenic by wheat, rape in creased with the total amountand concentration of species of arsenic in soils, whereas the fresh weight of wheat andrape decreased. As the added mass fraction of (As⁵⁺ reached 60 mg·kg^-1; As³⁺ reached 20mg·kg^-1) the fresh weight of wheat markedly decreased. As the added mass fraction of (As⁵⁺ reached 80 mg·kg^-1; As³⁺ reached 40 mg·kg^-1) the fresh weight of rape markedlydecreased.4. Arsenic can be transformed into volatilizable arsenic by microorganism in the soiland transferred to atmosphere. The authors focused on the effect of differentenvironmental conditions on the biovolatilization of arsenic, and the results showed thatthe velocity of biovolatilization increased with the increase of arsenic concentration in thesoil. The moisture content in the soil also affected the biovolatilization of arsenic. Thevelocity of biovolatilization became lower if the moisture was high enough or low enoughin the soil. And the temperature in the ambient also affected the biovolatilization ofarsenic. The variety of crop (winter wheat or rape) is not affected the biovolatilization ofarsenic.5. Rhizobox method was used to study the relationships between the total quantity ofarsenic and five soil enzymatic acitvities(urease, catalase, neutralphosphatsae,dehydrogenase, Polyphenol oxidase,) in rhizosphere soil of winter wheat and rape. Theresult showed that uraseaer activities are inhibited in 240 mg·kg^-1As(â…¤), between 0～160mg·kg^-1As(â…¤) uraseaer activities stimulated. And uraseaer activities are inhibited in80～240 mg·kg^-1As(â…¢), between 0～80 mg mg·kg^-1 AS(â…¤) uraseaer activities stimulated.the uraseaer activities in rhizosphere of winter wheat was more than that rhizosphere ofrape as for the same arsenic content. Polyphenol oxidase, dehydrogenase activities mainlyexhibited inhibiting effect in 0～240mg·kg^-1arsenic, along with concentration of aresnicincreasing, polyphenol oxidase, dehydrogenase activities debase, and exhibited asignificant negative correlation. Neutralphosphatsae activities are inhibited in 80～240mg·kg^-1As(â…¤), between 0～80 mg·kg^-1As(â…¤) uraseaer activities stimulated. Andneutralphosphatsae activities are inhibited in 0～240 mg·kg^-1 As(â…¢). Catalase activitiesmainly exhibited stimulating effect in 0～240mg·kg^-1arsenic, along with concentration ofaresnic increasing, catalase activities increase, and exhibited a significant positivecorrelation.6. Soil basal respiration, soil biomass carbon, soil biomass nitrogen, metabolicquotient are important indexes of soil microbiological, the relate to physical-chemicalcharacter of soil. The dehydrogenase activity is the important sensitivity index ofmicrobial community and activityon soil. Different added arsenic concentration rangingfrom 0 mg·kg^-1 to 240 mg·kg^-1 effect on soil basal respiration, soil biomass carbon, soilbiomass nitrogen, metabolic quotient and dehydrogenase activity. The experiment wasstudied in house incubated rhizosphere soil of winter wheat and rape samples. The resultsshowed as follows: soil biomass carbon, soil biomass nitrogen decrease with extendincubated times. Soil basal respiration was increased with increased concentration ofarsenic. Soil metabolic quotient were changed with different incubated time. Soildehydrogenase significantly decrease under arsenic stress. The correlation analysisindicated that soil added arsenic content different correlation with soil biomass carbon,dehydrogenase activity in different in cubation times. The available arsenic and soil biomass carbon, soil biomass nitrogen, microbial etabolic quotient and dehydrogenaseactivity exhibited a significant negative correlation.7. Under laboratory conditions, with traditional cultivation method, effect of arsenicat different concentrations on microbial populations and major microbial physiologicalgroups in wheat and rape soil were studied. The results showed that along withconcentration of aresnic increasing, the population of microorganism and the growth ofbacteria, actinomyces and fungi in soil were debase, the order of the aerobicmicroorganisms inhibited by arsenic actinomycetes＞fungi＞bacteria. The analysis ofmicrobial populations in the rhizosphere of winter wheat and rape demonstrated that thenumber of bacteria was significantly higher than ones of actinomycetes and fungi. Theanalysis of microbial populations in the rhizosphere of winter wheat and rapedemonstrated that the number of bacteria was significantly higher than ones ofactinomycetes and fungi, It indicated that the effect of rape on microbes was mostremarkable. The numbers of bacteria in rhizosphere and non-rhizosphere soil wassignificant negative correlation with the soil arsenic concentrations. The numbers ofbacteria in non-rhizosphere soil ase significant negative correlation with the availablearsenic concentrations. Through analysis, the result of statistics was not show the numbersof actinomycetes had remarkable negative correlation with the soil arsenic concentrations,but the actinomycetes' numbers declined with increase of the soil arsenicconcentrations.