Characteristic Effect Of Two Organic Acids On Adsorption-desorption Zn²⁺?Cd²⁺ In Tea Plantation Soil Aggregates

There is more than 27 thousands km² tea planting in our country,production of which in the top of world is a mainstay industry of south China.Quality of the tea is markedly affected by its growth soil environment.In the process of increasing production of tea producing areas,the soil heavy metal content is increased year by year by extensive management and fertilization,which directly threatened the safety of tea planting.Citric acid as a kind of low molecular weight organic acids,itself is biodegradable,and can form a stable complex compound with heavy metals,particularly,it flexibly adjust the adsorption-desorption behavior of soil heavy metal to some extent.Citric acid is an important energy of microbial growth and has direct impacts on soil microbial quantity and types,which can be formed stable chelate with heavy metal.EDTA,as synthetic organic acids can form stable compounds with most of the heavy metals.So citric acid and EDTA have better application prospects in the repair of heavy metal contaminated soil.Therefore,this article selects the tea plantations purple soil as the research objects in Mengding mountain of Ya'an,Sichuan,under the condition of adding different concentration of citric acid and EDTA,using the method of mass culture and equilibrium adsorption-desorption test,to study the influence of features and differences of two kinds of organic acid on adsorption-desorption single and coexistence of Zn²⁺and Cd²⁺on the original soil and different size of soil aggregate?2⁰.25mm,0.25⁰.05mm,0.05⁰.002mm,<0.002mm?.Which's results can predict plant tea area soil Zn²⁺and Cd²⁺environmental capacity and potential pollution condition,and provides the theoretical basis for the reasonable choice of compound pollution prevention and cure.The main conclusions are as follows:?1?After adding organic acid,soil aggregate for a single state Zn²⁺and Cd²⁺adsorption characteristics as follows:when the concentration and particle size of two kinds of organic acid fixed phase at the same time,the Zn²⁺and Cd²⁺ion concentration,the greater to the adsorption quantity of the original soil and different particle sizes soil aggregate.When the concentration of Zn²⁺and Cd²⁺was constant,the concentration of citric acid and EDTA was fixed,and the order of soil aggregates on Zn²⁺adsorption as follows:?<0.002mm?>0.05⁰.002mm>original soil>2⁰.25mm>0.25⁰.05mm.While the order of Cd²⁺as follows:<0.002mm and 0.05⁰.002mm>original soil>0.25⁰.05mm>2⁰.25mm..The changes of Zn²⁺adsorption capacity under different concentrations of organic acids were consistent with that of Cd²⁺,wherein the concentration of citric acid was 0.1mmol/L,and Zn²⁺adsorption reached its peak.When the heavy metal ions were the same(Zn²⁺,Cd²⁺),the adsorption capacity of heavy metal ions by the original soil and each particle size soil aggregate under the action of citric acid is greater than that of the original soil and each particle size soil aggregate under the action of EDTA.The Langmuir equation and the Freundlich equation had reached a very significant level in their adsorption and in fitting processes,which shows that the Freundlich equation had a better fitting effect.?2?When organic acids and EDTA were added,the adsorption capacity of Zn²⁺and Cd²⁺in the soil aggregates as follows:within the same organic acid concentration and the particle size,the initial concentration of heavy metal ions(Zn²⁺,Cd²⁺)is increased,the easy desorption rate increased.When the same particle size and the initial heavy metal ion concentration were the same,desorption rate of heavy metal ions tended to increase within the increase of organic acid concentration.When the initial concentration of heavy metal ions is a constant,the desorption rate of Zn²⁺under the same concentration of organic acids is:0.25⁰.005mm>2⁰.25mm>original soil>0.05⁰.002mm>?<0.002mm?,as for Cd²⁺,the order of desorption rate change is 2⁰.25mm>0.25⁰.005mm>original soil>0.05⁰.002mm>?<0.002mm?.When the organic acid concentration is 10mmol/L,the desorption rate of Zn²⁺under citric acid is lower than that of EDTA,and the desorption rate of Cd²⁺under citric acid is higher than that of EDTA.Under the action of organic acids,the desorption rate of Cd²⁺is higher than the desorption rate of Zn²⁺.?3?With the addition of organic acid?EDTA or citric acid?in the Zn²⁺and Cd²⁺coexistence system,the adsorption amount of Zn²⁺and Cd²⁺on original soil,the adsorption capacity of Zn²⁺and Cd²⁺was the best in small size,the second was the original soil and the weakest in the large particle size.With the increase of the concentration of Zn²⁺and Cd²⁺,the accelerating effect of citric acid on its adsorption will gradually weaken.Low concentration of citric acid promoted adsorption better than EDTA,and high concentration of EDTA inhibited the adsorption of citric acid more than citric acid.Using the Langmuir and Freundlich to describe the adsorption capacity of Zn²⁺and Cd²⁺by bulk soil and different size aggregates at organic acid,the Freundlich was superior to Langmiur.In coexistence system,desorption rate of Zn²⁺and Cd²⁺by bulk soil and different size aggregates at organic acid showed as follows:0.25⁰.05mm>2⁰.25 mm>original soil>0.05⁰.002mm>?<0.002mm?,and the desorption rate of Cd²⁺is higher than that of Zn²⁺.?4?Citric acid and EDTA will promote adsorption in low concentration,while promote desorption in high concentration for single and coexisting Zn and Cd,but the competitive adsorption weakens the dual effects.Citric acid promotes better adsorption than EDTA,and EDTA is better than citric acid in desorption.This article selects the Sichuan tea plantation soil,study the influence of the adsorption and desorption characteristics of Zn and Cd under the action of two kinds of organic acid?citric acid and EDTA?of the tea garden original soil and different soil aggregate particle size,aims to explore buffer capacity of the competition or noncompetitive adsorption effect of Zn,Cd on whole and part of the tea garden soil,and preliminary clarify effectiveness and validity of regulating and controling stability and mobility of Zn,Cd by improvement soil aggregate structure and using two kinds of organic acid,In addition,the practical scope and effect of the regulation mechanism are further discussed in article.The study results can be used as scientific basis for perfecting the mechanism of a single or joint repair,guiding to reduce potential risks of single and compound pollution of heavy metals in tea garden soil,in order to better protect the ecological security in Mengding Mountains.