The Effect Of Soil Active Organic Component On The Chemical Speciation Of Cu²⁺ And Phosphate

Soil active organic components are ubiquitous in soil environments.They contain numerous functional groups and thus play an important role in regulating the bioavailability of heavy metal ions and nutrient ions by affecting their chemical speciation directly or indirectly.The interactions between ions and soil active organic components have been investigated before.Due to the heterogeneity of HS,only the average characteristics of the overall binding could be obtained,and this thwarts the interpretation because averages of ion binding characteristics over two different site-types?carboxylic and phenolic?were obtained.In the present study,the effect of soil active organic components?Humic acid,HA;Low-molecular weight organic matter,LMWOM?on the chemical speciation of heavy metal ion(Cu²⁺)and nutrient ion?phosphate?was investigated by combining batch adsorption experiment,chemical speciation model?NICA model and CD-MUSIC model?,X-ray absorption fine structure?XAFS?spectroscopy,isothermal titration calorimetry?ITC?and attenuated total reflectance Fourier transform infrared spectroscopy?ATR-FTIR?.Site-type-specific information was obtained to developing a molecular picture of soil active organic component-ion interactions.The main results are as follows:?1?The speciation of Cu bound largely depended on the functional groups of HAs?JLHA and JGHA?.In the pH range of 4.0 to 6.0,Cu binding was controlled by carboxylic-type sites.The phenolic-type sites played an increasing role in Cu binding with increasing pH and Cu loading.For Cu binding to JLHA(Q_maxH,1=4.46 mol/kg,Q_maxH,2=1.38 mol/kg),the contribution of phenolic-type sites was less than 8%.The comparable amount of carboxylic-?2.97 mol/kg?and phenolic-type?2.86 mol/kg?sites on PAHA made that both carboxylic-and phenolic-type sites controlled the Cu binding.The fraction of phenolic-Cu was in the range of 15%-48%.The intrinsic affinity of phenolic-type sites to Cu(log^<sup>k_Cu,2=8.26/8.55)was much larger than that of carboxylic-type sites to Cu(logk^<sup><sub>Cu,1=3.82/4.65).Both carboxylic and phenolic conditional affinity spectra?CAS?shifted with decreasing pH toward lower values.The fact that the shift of phenolic distribution was larger than that of the carboxylic distribution led that the carboxylic CAS had a stronger affinity for all samples at pH 4-6.Cu binding took place preferentially on the sites with high affinities.The stability constants of HA-Cu decreased with increasing Cu loading.?2?The“universal nature”of the site-type-specific thermodynamic mechanism was obtained for Cu binding to the two types of functional groups on JLHA and PAHA.The site-type-specific thermodynamic parameters obtained by combining ITC measurements and NICA calculations revealed that Cu binding to deprotonated carboxylic-type sites was entropically driven and that to deprotonated phenolic-type sites was driven by both entropy and enthalpy.Copper binding to HA largely depended on the site-type and coordination environment,but the thermodynamic binding mechanisms for Cu binding to the specific site-types were similar for the different HAs studied.By comparing the site-type-specific thermodynamic parameters of HA-Cu complexation with those of LMWOM-Cu,the Cu coordination could be further specified.Bidentate phthalic-Cu,salicylic-Cu and catecholic-Cu structures made the dominating contributions to Cu binding to HAs.The monodentate complexes should be responsible for the minority of HA-Cu complexes.?3?The adsorption speciation of phosphate on goethite was influenced by pH,phosphate loading and Al substitution.With decreasing pH,increasing ionic strength and Al substitution,the charge density increased.Al substitution enhanced phosphate adsorption by about 7%,which was ascribed to the increasing site densities of?FeOH^-0.5 and?AlOH^-0.5 from 3.48 sites/nm² to 3.88 sites/nm² with decreasing ratio of?110?/?021?faces.The combination of CD-MUSIC modeling and ATR-FTIR/2D-COS showed that three types of phosphate complexes were formed on goethite:mononuclear monodentate nonprotonated complex?M_nM_d H₀?stabilized by hydrogen bonding to adjacent sites,binuclear bidentate nonprotonated complex?B_nB_d H₀?,and binuclear bidentate monoprotonated complex?B_nB_dH₁?.At low phosphate loading,B_nB_d H₀ and M_n M_d H₀ were preferentially formed on goethite surface.At high phosphate loading and low pH,B_nB_dH₁ was the dominant species.With increasing pH,M_nM_dH₀ became increasingly important.The fraction of bidentate complexes increased slightly with Al substitution.?4?The adsorption behavior of catechol?CA?,phthalic acid?PA?and salicylic acid?SA?,which were HS-analogue LMWOM with different functional groups,was significantly distinct.With increasing pH,the adsorbed amount of PA and SA on goethite decreased,while that of CA increased at first and then decreased.The adsorption capacity of CA,PA and SA on goethite at pH 6.0 followed the order:CA?1.65?mol/m²?>PA?1.28?mol/m²?>SA?0.57?mol/m²?,whereas the apparent affinity was:PA?35.34 L/mmol?>CA?13.61 L/mmol?>SA?5.80 L/mmol?.SA formed inner-sphere complex in a mononuclear monodentate configuration with carboxylate bound to the iron atom on goethite surface within pH 4-10.Outer-sphere or H-bond complex made the main contribution to PA adsorption on goethite over the pH range studied.With decreasing pH,bidentate inner-sphere PA complexes increased but were still in the minority.The adsorption of CA on goethite was by forming both monodentate and bidentate complexes.The latter was charge transfer complex with Fe?III?on goethite surface reduced to Fe?II?.The fraction of bidentate CA complex was affected by pH and the maximum was reached at around pH 8.?5?Compared with PA and SA,CA affected the phosphate adsorption on goethite more significantly.The effect of CA,PA and SA on phosphate adsorption on goethite varied with pH,LMWOM concentration as well as the mechanism of LMWOM adsorption on goethite.SA could hardly influence phosphate adsorption even at high SA concentration.The presence of PA had limited effect?<2%?on adsorbed phosphate by forming inner-sphere PA complexes in acidic conditions.A substantial decrease of phosphate adsorption was observed at high CA concentration and at around pH 8.The formation of bidentate charge transfer complex for CA adsorption on goethite played an important role in inhibiting phosphate adsorption.LMWOM affected phosphate adsorption by mainly declining B_nB_dH₁ phosphate complex.Due to the much higher affinity of goethite to phosphate,the presence of phosphate suppressed the adsorption of LMWOM significantly.In summary,environmental conditions such as pH and ion concentration,as well as functional groups of soil active organic components were the key factors affecting the chemical speciation of cations and anions.Carboxylic-type sites made the main contribution to influencing the adsorption behavior of ions at relatively low pH.With increasing pH,phenolic-type sites played important roles.The environmental effects of ions were controlled by their chemical speciation.Therefore,the results presented here could potentially provide fundamental information for understanding the fate of cations and anions in natural environments.It is also of great theoretical and practical significance to the remediation of soils contaminated by heavy metals and effective regulation of P fertilization.