The Acid-Base Buffering Capacity And Surface Complexation Model Of Red Soils In The Hilly Region Of Southeastern China

Red soil is an important soil type in southern China,but the acidification is becoming more and more serious at present,seriously affecting the local agricultural production and ecological balance.Although China has made a lot of progresses in the acidification and regulation of soils,the researches mainly referred constant charge soils in Europe and the United States,and were short of specialized research on the highly acidified soils with variable charge in China.The surface complexation model（SCM）studying complexation reaction between the metal oxide and ion can obtain the related complexation constants to describe the surface acid-base properties of oxides and explain the interfacial reaction mechanism.However,whether this SCM is also suitable for the analysis of acid-base buffer capacity of soil components and natural red soils needs further to study.In this paper,1-site/2-pK_aageneralized composite SCMwere applied to study the acid-base buffer capacity of the red soil.First of all,the main variable charge components,humic acid,goethite,hematite and kaolinite,were selected as the research model.As a result of being protonated and deprotonated under different acid-base conditions,the surface hydroxyl groups can buffer the pH changes of red soil.In this study,the soil components were titrated by the standard HCl and NaOH solution through the auto potentiometric titration under the controlled pH=2.9⁹.5,to study the surface charge of soil minerals and the point of charge pH_pzc.The X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,and N₂ desorption/adsorption isotherms（BET）were used to characterize the crystal structures,surface groups and specific surface areas of soil minerals.Based on the characterization data and titration curves,the acid-base properties of the minerals were analyzed by using1-site/2-pK_a surface complexation model,and can explain the acid-base buffer capacity of the minerals quantitatively.The Gran plot method,commonly used to determine the equivalence points,was applied to calculate the concentration（H_s）and density（D_s）of the surface active sites on the soil minerals.The acid-base equilibrium constants(pK_a^int)of soil minerals were obtained bythe linear extrapolationof pKa vs.Q curves,and the point of zero charge(pH_pzc)was calculated bythe following formula:pH_pzc=1/2(pK_a1^int+pK_a2^int).The differencesbetween the acid-base buffering capacity of kaolinite and iron oxides were quantified using these parameters,and the crystal structure and surface functional group structure were used to analyze the differences.Besides,the results showed the model-derived pH_pzc were close to those the experimental pH_pzc.Thus,it is feasible to apply the SCM to acquire the buffer capacity of the various charge components.According to the parameters,the surface chemical speciation of the various charge components at different pH were calculated by Visual Minteq software with the double layer model（DLM）to explain the mechanism of acid-base buffer behavior on the components surfaces.Based on the successful application of the SCM to four variable charge components,the acid-base titration method and the model calculation method were applied to five profiles of laterite soil samples from 0to 140 cm in Huizhou,Guangdong Province.The results also proved the SCM feasible.The change of acid-base buffering capacity from the to the top and bottom layer soil were compared,and the relationship between buffering capacity and its different constituents were analysed.Subsequently,SCM was applied again to the red soil samples at five sites（brick red soil in Hainan,red soil in Guangdong,red soil in Jiangxi,paddy soil in Zhejiang and paddy soil in Hubei）in order to explore the applicability of the model to different red soil samples and its buffering capacity influenced by its composition.The main conclusion are as follows:（1）The result of calculated values of pH_pzc were nearly equal with the experimental values,which showed that the 1-site/2-pKa generalized composite SCM was feasible to the various charge components of the red soil.In addition,the order of H_s wasas follows:Humic acid(1.5067 mol·kg^-1)>Goethite(0.0313mol·kg^-1)>Kaolinite(0.0272mol·kg^-1)>Hematite(0.0188mol·kg^-1),and the order of pH_pzc was as follows Hematite（9.27）>Humic acid（6.45）>goethite（5.14）>kaolinite（4.82）.The results demonstrated that humic acid and goethite had abundant surface active groups,which can increase the acidification buffer sites of the red soils.And hematite with the higher pH_pzc can increase the pH_pzc of the red soils.（2）The comparison of both pH_pzc shown that SCM was also suitable for analyse the acid-base buffering capacity of natural red soils at different profiles.However,the acid-base buffering capacities of the two top soil samples were significantly higher than the bottom three soil samples.The correlation results shown the organic matter content was significantly positively correlated with the concentration of surface active sites H_sand better positively correlated with pH_pzc,which indicated that the organic matter played a leading role in the process of buffer acidification in red soil.However,the hematie content was significantly negatively correlated with H_s,which indicated hematite decreased the H_s.Furthermore,because kaolinite is the major soil component,the pH_pzc of the red soils were mainly determined by the dominant mineral.（3）When the 1-site/2-pK_a generalized composite SCM was applied again to the acid-base buffering capacity of red soil samples from six sites in the southeastern hills,the pH_pzczc calculated from the SCM calculation was consistent with those from acid-base potentiometric titration experiments.It was suitable for studying the surface acid-base buffering ability of the red soil samples form the five sites.When the soil buffering capacity was related to its components,the phenomenon was discovered as follows:the red soil of Hainan Chengmai had a higher pH_pzcdue to the higher content of hematite;the paddy Soil of Jiangmen Guangdong had higherH_sbecause of its higher organic matter;In addition,it was found that the rice plants in Jiaxing,Zhejiang and Xianning,Hubei Province still had higher calcium carbonate,and the pH_pzc,H_s,and D_s were relatively high.（4）The speciation（≡SOH₂⁺、≡SOH、≡SO^-）distributions of the red soils surfaces at different pH were calculated with the DLM,which can explain the mechanism of acid-base buffering behavior on the red soils surfaces.The simulations showed that H_s,pH_pzc,pK_a1^int,and pK_a2^int can quantify different acid-base buffering properties of the red soils,and the acid-base buffering capacity were determined by these parameters together,which explains the significance of these parameters for determining their acid-base buffering capacity.