Effects Of Humic Acids On The Speciation And Bioavailability Of Arsenic In Soils

Arsenic(As),as a typical metalloid pollutant,its pollution is more and more serious.It has been paid wide attention to As pollution and its curb.The environmental damage of soil As is not only related to its total dose,but also closely related to its speciation in the soil.The speciation distribution and biological activity of soil As is influenced by many environmental factors,and organic matter is one of the most important influence factors.Humic acids(HAs)are the main composition of organic fertilizer and soil organic matter.Humic acid(HA)and fulvic acid(FA)are key active components to influence the speciation and activity of soil As.HAs have many active perssads such as carboxyl,hydroxyl,amino,quinone,sulphur,which have significant effects on adsorption-desorption and forms transformation of soil As,through complex mechanism such as the surface adsorption,chelation,and ion exchange.Soil heavy metals not only caused environmental damage through the food chain,but also a direct harm to human body by means of direct exposure,called the bioaccessibility of heavy metals.This study planned to analyzes how the HAs influence environmental behavior(such as adsorption-desorption,speciation transformation)of soil As,to explore effects of HA and FA content and its proportion to the bioavailability(phytoavailability and bioaccessibility)of soil As,to reveal the relationship of bioavailability and speciation of soil As.An isothermal adsorption-desorption test and a sequential extraction procedure were carried out to study the adsorption-desorption characteristics and speciation of soil As.The chemical extraction and the in vitro simulation method were used to stydy the phytoavailability and bioaccessibility of soil As.The main research results obtained as follows:The influence of humic acids(HAs)on the adsorption and desorption of soil As was closely related to the types,content and the proportion of active component HAs.The isothermal adsorption of the soil As could well fitted by Langmuir equation,which correlation coefficient(R2)was between 0.971 and 0.996.HA could promote the soil adsorption of As,increasing the maximum adsorption capacity of soil As.With the increase of adding amount,the maximum adsorption capacity increased gradually.When HA content was the highest(5% C),the maximum adsorptioncapacity was 2162.29 mg·kg-1,which was 2 times of control group(1052.95 mg·kg-1).FA could inhibit soil adsorption of As,reducing the maximum adsorption quantity of soil As.It has the lowest maximum adsorption capacity when FA content was the highest(5% C).Under the same carbon content(1%)of the two components mixed,HAs could promote the adsorption of soil As When HA/FA ratios ≥5/5 and decrease adsorption when HA/FA ratios ≤3/7.HA also could inhibit the desorption of soil As.With the increase of adding amount of HA,the inhibition of desorption enhanced gradually.FA promoted the desorption of soil As,increasing the environmental risk of soil As.HAs could promote desorption of soil As when HA/FA ratios ≤5/5 and inhibit desorption when HA/FA ratios ≥7/3.From the view of adsorptive fixation and desorption release,HA increased soil adsorption of As and inhibit the release of soil As,which was beneficial to reduce the harm of soil As.However,FA inhibited the adsorption and promoted desorption of soil As,increasing the environmental hazards of soil As.It was transformed into different speciation immediately after exogenous As got into soils.The transformation was violent at 15 days before,and tended to balance and stability in a certain level after 30 days.FA promoted the transformation of exogenous As from Ex-As,Ca-As to Fe-As,Al-As and Res-As.While HA promoted Ex-As,Ca-As,Al-As transformed into Fe-As and Res-As.The two components of HAs had significant effects on As species in soils,and the influence degree and directions were connected with HAs concentration,proportion of the two components and exogenous As content.FA and HA which carbon content ≤1% could increase the allocation proportion of Ex-As.It had the highest As proportion of 6.29% at FA adding amount of 1%C.While HA which carbon content ≥3%could decrease the content of Ex-As and there was the lowest As proportion of 1.20% at HA adding amount of 5%C.FA could observably promote the transformation of soil As from Al-As and Fe-As forms to Res-As species,and the effects enhanced along with the increase of the FA dosage.HA showed weaker or even reversed roles at the highest usage of 5%C in affecting such a transformation process.Under the condition of the same amount of HAs,when HA/FA ratio was 7/3,HAs could decrease the distribution ratio of Ex-As,but when HA /FA ≤3/7 HAs could increase the distribution ratio of Ex-As.HAs with different HA/FA ratios always decreased the distribution ratio of Al-As and Fe-As and increased the Res-As ratio significantly.Al-As and Fe-As had the lowest distribution of21.97% and 42.73% while Res-As had the highest percentage of 24.51% at an HA/FA ratio of 3/7.HAs had great influence on the bioavailability of soil As.Different FA content had different effects on the phytoavailability of soil As.FA at low usage(≤3% C)could increase the phytoavailability of soil As.However,FA at higher usage(5% C)could reduce the phytoavailability of soil As.HA could observably reduce the phytoavailability of soil As,and the effects enhanced HAs with different HA/FA ratios always promoted the phytoavailability of soil As.The content ofphytoavailability As decreased along with the increase of the HA/FA ratio.Ex-As and Ca-As forms were found to have significant positive contribution to the phytoavailability of soil As,with Ex-As being greater than Ca-As per unit.HAs had great influence on the bioaccessibility of soil As.FA could increase the bioaccessibility of soil As in both gastric and small intestinal phase.With the increase of FA usage,the bioaccessibility of soil As in gastric phase increased then decreased,and the bioaccessibility of soil As in small intestinal phase always increased gradually.HA had different effects from FA on the bioaccessibility of soil As in both gastric and small intestinal phases.HA could reduce the bioaccessibility of soil As in gastric phase at an application dosage of ≤1%C as PBET method used.The lowest bioaccessibility was 24.78%.HA could increase the bioaccessibility of soil As in small intestinal phase as IVG method used.However,HA,carbon content >0.5%C,could decrease the bioaccessibility of soil As in small intestinal phase as PBET method used.HAs with different HA/FA ratios always enhanced the bioaccessibility of soil As in both gastric and small intestinal phases at a constant usage of 1% C.The bioavailability and bioaccessibility of soil As was strongly linked to As speciation in soil as influenced by HAs.It was the same to phytoavailability,the species which were found to have significant positive contribution to the bioaccessibility of soil As were Ex-As and Ca-As forms.There were significant correlation between bioaccessibility As and phytoavailability As in soil under the influence of HAs.When dealing with As polluted soils using organic materials,the content and proportion of active component of HAs should be fully considered to reduce the content of Ex-As and Ca-As,to reduce the potentially harmful to plants and human health.