Selective And Quantitative Adsorption Mechanisms Of Soil Humic Substance On Multi-component Minerals

Humic substances(HSs),iron oxides and phyllosilicate minerals are ubiquitous in soil systems.They are key constituents of soils and sediments as important active components in nature systems.Most of them possess a high surface area and a large amount of reactive functional groups can form of organic-inorganic complexes.The formation processe of organic and inorganic complexes is the basic process of soil aggregation formation and it benefits for promoting the stability of soil organic matter.However,many studies of interactions between HS and minerals focus on the qualitative,less of information to quantify the contribution of mechanisms and also the selective adsorption mechanisms of HS on different mineral surfaces.Humic acid(JGHA)and fulvic acid(JGFA)were extracted from upper horizon of meadow soil of Jiugong Mountain in Hubei.Kaolinite(KAO)and montmorillonite(MON)used in this study were purchased from the Clay Minerals Society Source Clays Repository to represent for phyllosilicate minerals.Kaolinite-goethite mixture(KGM)and association(KGA)were synthesised in laboratory.A combination of isothermal titration calorimetric(ITC),attenuated total reflectance Fourier Transform(ATR FTIR),high performance liquid chromatography(HPSEC)and two dimensional correlation spectroscopy analysis(2D-Cos)was used to investigate the thermodynamics and kinetic adsorption behaviors of HS on clay minerals.The quantitative mechanisms of soil HS adsorption on phyllosilicate minerals was obtained.The selective adsorption behaviors of HS in the presence of different mineral surfaces was revealed.The main results as follows:1.The adsorption capacities of JGHA on phyllosilicate minerals were decreased with p H increasing which revealed the contribution of adsorption mechanisms at different p H.At p H 4.0,the largest adsorption amount of JGHA on KAO(0.48 ?mol/m2)was larger than that of MON(0.39 ?mol/m2)and the adsorption affinity of KAO(ln KL: 13.8)was larger than that of MON(ln KL: 14.7).While,the adsorption amount of JGHA on KAO(0.28 and 0.23 ?mol/m2)was smaller than that of MON(0.37 and 0.33 ?mol/m2)at p H 6.0 and 8.0 and also the adsorption affinity(KAO: ln KL 12.6-11.4;MON: ln KL 13.0-12.9).At low p H,the estimated contribution of JGHA adsorbed on KAO through ligand exchange and electrostatic attraction was 26-32% and on MON was 22-26%.At p H 4.0,the positive charges on KAO were larger than that on MON;thus,there were more contribution of ligand exchange and electrostatic attraction driven JGHA adsorption on KAO than that on MON.At high p H,the increasing electrostatic repulsion inhibited the adsorption and enhanced the importance of hydrophobic effect.Furthermore,the hydrophobic groups on the surface of MON were larger than that of KAO which caused the stronger adsorption capacities of JGHA on MON at high p H.2.The adsorption capacities of JGFA on phyllosilicate minerals were increased frist and then decreased with p H increasing which enhanced the formation of different complexes.The larger adsorption amount of JGFA on KAO was 2.65 ?mol/m2 at p H 5.5 and MON was 2.32 ?mol/m2 at p H 6.5.At p H 4.5-5.5,the adsorption amount on MON increased slowly with p H increasing(?ads: 0.02 ?mol/m2);At p H 4.5-6.5,the adsorption amount on KAO increased faster than on MON with p H increasing(?ads: 0.07 ?mol/m2).At same p H,the adsorption amount of JGFA on KAO larger than that on MON.The-COOH of JGFA interacted with phyllosilicate minerals to form inner-sphere complexes and-OH to form outer-sphere complexes.At low p H range,the formed inner-sphere complexes increased first and then decreased with p H increasing and outer-sphere complexes increased with p H increasing.At high p H(p H > 7.0),JGFA interacted with phyllosilicate minerals to form outersphere complexes and the formed outer-sphere complexes decreased with p H increasing.3.The adsorption of JGHA on kaolinite-goethite binary systems were distinctly affected by p H which suggested the selective adsorption behaviors of JGHA with different molar mass fractions on different minerals.The adsorption capacities of JGHA on kaolinite-goethite binary systems decreased with p H increasing.At p H 4.0,the adsorption amount of JGHA on KGA(0.22 ?mol/m2)and KGM(0.24 ?mol/m2)was similar.The surface charge of KGA(210 m C/m2)was similar to KGM(204 m C/m2)at p H 4.0 which suppressed the difference of the adsorption that driving by ligand exchange and electrostatic attraction.At p H 6.0 and 8.0,the adsorption capacities of JGHA on KGM was stronger than that on KGA.The surface area was 16% of KAO in KGA,which was less than that in KGM(31%).The results caused higher adsorption of KGM on hydrophobic sites.At low p H,the high(34.3-12.9 kDa)and low molar mass(12.9-0.4 kDa)fractions of JGHA was preferential adsorbed by kaolinite-goethite binarys;at high p H,the < 12.9 kDa molar mass and hydrophobic fractions was adsorbed by binarys.The > 12.9 kDa molar mass fractions of JGHA was adsorbed on edge face,Al basal face of KAO and surface of GOE which was driven by ligand exchange and electrostatic attraction;the < 12.9 kDa molar mass fractions adsorbed on Si basal face of KAO and driven by hydrophobic effect.4.The adsorption capacity of JGFA on KGA and KGM were increased first and then decreased with p H increasing which consisted with JGFA adsorption on KAO.The largest adsorption amount(2.09 ?mol/m2)of JGFA on KGA was at p H 5.5 and on KGM(2.68 ?mol/m2)at p H 4.5.The change adsorption amount of JGFA on KAO(?ads = adsmax-adsmin)was 0.1 ?mol/m2 and on KGA and KGM was 0.04 and 0.02?mol/m2,respectively.The effect of p H of JGFA adsorption on binarys was weaker than that on KAO.At low p H,the dissolved-COOH of JGFA interacted with variable charge sites of KAO and GOE in binarys to form inner-sphere complexes and-OH to form outer-sphere complexes.At high p H,the formation of inner-sphere complexes was largely inhibited due to the ATR peaks of carboxyl groups disappeared.The adsorption affinity of-COOH on binarys was stronger than the adsorption of-OH.5.The adsorption of JGHA and JGFA on phyllosilicate minerals and GOE-KAO binasy systems was spontaneously,while the thermodynamics adsorption behaviors were different.The adsorption of JGHA on phyllosilicate minerals and binarys was exothermic.The entropy change(T?S)of JGHA adsorption on binarys was-545 ~-977 k J/mol/K and on phyllosilicate minerals was-59.5 ~-64.4 k J/mol/K.The adsorption of JGHA was driven by enthalpy.The thermodynamics adsorption behaviors of JGFA on binarys were significance dependent on p H.At p H 4.0,the adsorption of JGFA on phyllosilicate minerals was endothermic first and then exothermic and thus the adsorption driven by enthalpy-entropy(?H < 0,T?S > 0),while on binarys was only exothermic and driven by enthalpy.At p H 6.0 and 8.0,the adsorption on phyllosilicate minerals and binarys was exothermic and driven by enthalpy-entropy.The T?S of JGFA adsorbed on binarys was 16.5 ~-16.2 k J/mol/K and on phyllosilicate minerals was 9.7 ~-7.7 k J/mol/K,which was distinctly smaller than that of JGHA.The molar mass of JGHA(28 kDa)was ten times larger than that of JGFA(2.4 kDa).The smaller malor mass of JGFA caused entropy-increasing when JGFA adsorbed on mineral surfaces.Furthermore,the formation of inner-sphere will take off water molecule which also lead entropy-increasing.