| The soil layer located at the top of vadose zone has a great retention effect on the migration of Cr(Ⅵ)to groundwater,and the soil organic matter is considered as the most important factor accounting for this process.Soil humus,as the main component of soil organic matter,is highly chemical heterogeneous,and different soil humus fractions are quite different on molecular structural characteristics and functional group compositions.In order to reveal the influence of the chemical heterogeneous of soil humus on Cr(Ⅵ)retention,it is urgent to investigate the retention mechanisms of Cr(Ⅵ)by functional groups of different humus fractions,which is of great significance for further understanding the migration and transformation of Cr(Ⅵ)in vadose zone.In this study,a commercial humic acid(HA)and the humic acid(HA)and humin(HM)extracted from a typical soil sample were employed for revealing the mechanisms of functional groups for Cr(Ⅵ)retention by undissolved HA and HM under p H from 1to 4,and various spectroscopic techniques and a two-dimensional correlation spectroscopic analysis method were utilized.The main conclusions are listed as following:(1)Based on two-dimensional correlation spectroscopic analysis,a novel method of three-dimensional hetero-spectral correlation analysis was proposed for establishing a direct correlation analysis among three different types of spectroscopies,achieving a deeper data mining among various types of spectroscopies.Utilizing the novel method,free carboxyl,dissociated carboxyl,phenol and hydroxyl were identified to be the main functional groups of undissolved HA involved in Cr(Ⅵ)retention,which were more likely to located at aromatic domains of HA.(2)The retention of Cr(Ⅵ)by undissolved HA followed an adsorption-complexation-reduction mechanism,where Cr(Ⅵ)was firstly adsorbed onto HA by molecular diffusion and ion exchange,forming ion exchangeable Cr(Ⅵ),then the ion exchangeable Cr(Ⅵ)was chelated by carboxylic groups,forming complexed Cr(Ⅵ),and at last the complexed Cr(Ⅵ)was reduced to Cr(III)by adjacent reductive functional groups,such as phenol and hydroxyl.Besides,phenol,hydroxyl and methyl were the main electron donors for Cr(Ⅵ)reduction by undissolved HA instead of carboxylic groups,which mainly participated in chromium complexation.(3)Based on the mechanism of undissolved HA functional groups for Cr(Ⅵ)retention,an adsorption-complexation-reduction multi-step kinetic model was developed,and it fitted the experimental data well.Especially for the data of reduced Cr(III),the non-linear fitting optimization index R~*reached as high as 0.98,which was far more higher than that of pseudo-first order and pseudo-second order kinetic models(0.79 and 0.84 respectively).(4)The retention of Cr(Ⅵ)by HM followed the adsorption-complexation-reduction mechanism as well,but the adsorption rate and capacity of Cr(Ⅵ)by HM was much lower than that by undissolved HA.This was mainly caused by the high cellulose content of HM,which had a higher content of low polar oxygen-containing functional groups,such as hydroxyl and ether,and these functional groups in cellulose were quite resistant to Cr(Ⅵ)oxidation.(5)The HM linked to iron(HMi)and HM linked to clay(HMc)had similar molecular structures with undissolved HA,which can be considered as the same kind of humus with HA,and the reduction rate constants of Cr(Ⅵ)by these humus fractions were strong positively correlated with the content of phenol and polar carbon proportions(R~2>0.98),however,poorly correlated with aromaticity and aliphaticity(R~2=0.21).The property of HM residua(HMr)was quite different from other humus fractions,which had a high content of crystalline cellulose,and thus it reflected a non-linear character compared with other humus fractions. |
