KMS Institute of soil and water conservation Chinese Academy of Sciences
|Alternative Title||The Effect of Soil Active Organic Component on the Chemical Speciation of Cu2+ and Phosphate|
|Place of Conferral||陕西杨凌|
|Keyword||腐殖酸 低分子有机物 重金属 磷酸盐 化学形态|
（1）不同来源HA（JLHA和PAHA）的官能团组成特性决定着Cu的吸附形态。在所研究的pH范围内，Cu主要与HA羧基结合，酚羟基的贡献随pH升高、Cu浓度增大而增加。对于羧基含量（4.46 mol/kg）远高于酚羟基（1.38 mol/kg）的JLHA，酚羟基结合态Cu 低于8%；PAHA羧基（2.97 mol/kg）和酚羟基（2.86 mol/kg）含量相当，酚羟基结合态Cu可达15%~48%。HA酚羟基对Cu的内在亲和常数（logkCu,2=8.26/8.55）远大于羧基（logkCu,1=3.82/4.65）。随pH降低，羧基和酚羟基对Cu的亲和常数均降低，酚羟基降低更显著，其值甚至低于羧基，使Cu更易与羧基结合。Cu优先占据羧基和酚羟基的高亲和位点，因此低Cu浓度下形成的HA-Cu络合物更稳定。
（2）JLHA和PAHA特定官能团对Cu的内在吸附机制相同。联合ITC技术与NICA模型计算，获得Cu与HA去质子化的羧基和酚羟基结合的热力学参数表明，Cu与羧基结合是吸热（DHCuL,1 = 5.92~12.03 kJ/mol）、熵增（TDSCuL,1 = 29.98~36.33 kJ/mol）过程，受熵驱动；与酚羟基结合为放热（DHCuL,2 = -17.29~-19.69 kJ/mol）、熵增（TDSCuL,2=27.43~31.33 kJ/mol）的熵焓双驱动过程。该参数与LMWOM结合Cu的热力学参数对比，推断出Cu与HA羧基和酚羟基结合主要形成邻苯二甲酸盐-Cu、水杨酸盐-Cu和邻苯二酚-Cu等双齿配位结构以及少量单齿络合物。
（3）pH、磷酸盐浓度和铝同晶替代等因素不同程度地影响磷酸盐在针铁矿表面的吸附行为和吸附形态。随pH降低、离子强度增大、铝同晶替代量增加，针铁矿表面的正电荷密度增大。铝同晶替代使针铁矿(110)/(021)面比例减小，导致单配位基位点密度从3.48 sites/nm2增加至3.88 sites/nm2，使磷酸盐吸附密度增加约7%。CD-MUSIC模型联合ATR-FTIR光谱表明磷酸盐在针铁矿表面形成单齿单核去质子化（MnMdH0）、双齿双核去质子化（BnBdH0）和双齿双核单质子化（BnBdH1）三种内圈络合物，其中MnMdH0易与相邻位点形成氢键。低磷浓度下优先形成BnBdH0或MnMdH0络合物，高磷浓度、低pH条件下形成BnBdH1络合物；随pH升高，络合物由双齿向单齿转化；铝同晶替代使双齿络合物比例略微增加。
（4）邻苯二酚—CA、邻苯二甲酸—PA和水杨酸—SA羧基和酚羟基的组成不同，在针铁矿表面吸附特性差异较大。PA和SA在针铁矿表面的吸附均随pH升高而降低；CA吸附则先增加后降低，pH 8左右最高（1.85 μmol/m2）。pH 6.0时，CA、PA和SA在针铁矿表面的饱和吸附量为：CA（1.65 mmol/m2）> PA（1.28 mmol/m2）> SA（0.57 mmol/m2），表观亲和常数则PA>（35.34 L/mmol）> CA（13.61 L/mmol）> SA（5.80 L/mmol）。在pH 4~10范围内，SA在针铁矿表面形成较弱的单齿单核络合物；PA主要形成外圈或氢键络合物，低pH时存在少量内圈络合物；CA形成单齿和双齿络合物，后者还原针铁矿的Fe(III)，随pH升高，双齿（电荷转移）络合物的比例先增加后降低。
（5）与PA和SA相比，CA影响磷酸盐在针铁矿表面吸附更显著。LMWOM在针铁矿表面的配位机制决定了其对磷酸盐吸附的影响程度。SA在针铁矿表面配位较弱，不影响磷酸盐吸附；低pH时，PA形成少量内圈络合物，对磷酸盐吸附有微弱影响（< 2%）；CA对磷酸盐吸附的影响随pH升高先增大后减小，pH 8左右磷酸盐吸附量降低高达20%，原因可能有二：1）CA与针铁矿形成了稳定络合物，2）形成的双齿（电荷转移）络合物降低了针铁矿表面正电荷量。LMWOM主要影响针铁矿表面BnBdH1形态的磷酸盐，磷酸盐对针铁矿表面LMWOM吸附量和吸附形态均有显著影响。
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 (QmaxH,1=4.46 mol/kg, QmaxH,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 (logkCu,2=8.26/8.55) was much larger than that of carboxylic-type sites to Cu (logkCu,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 (MnMdH0) stabilized by hydrogen bonding to adjacent sites, binuclear bidentate nonprotonated complex (BnBdH0), and binuclear bidentate monoprotonated complex (BnBdH1). At low phosphate loading, BnBdH0 and MnMdH0 were preferentially formed on goethite surface. At high phosphate loading and low pH, BnBdH1 was the dominant species. With increasing pH, MnMdH0 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 mmol/m2) > PA (1.28 mmol/m2) >SA (0.57 mmol/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 BnBdH1 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.
|MOST Discipline Catalogue||农学|
|徐晋玲. 土壤有机活性组分对Cu2+、磷酸盐形态转化的影响机制[D]. 陕西杨凌. 中国科学院大学(中国科学院教育部水土保持与生态环境研究中心),2019.|
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