Contribution Of Minerals To Typical Organic Pollutants Sorption In Soils:A Quantification Method Study

An in-depth understanding of the sorption behavior of organic pollutants(OPs)in soils is an important prerequisite for predicting their biodegradability,bioavailability and environmental risks in soil.As a dual sorbent,soil is highly heterogeneous and mainly composed of soil organic matter(SOM)and minerals.As previous reports,when SOM and minerals were associated,the coating of clay minerals had weakened the partition function of SOM or blocked some sorption sites within SOM,resulting in a negative and indirect contribution to sorption of OPs;while other researchers found the interaction between SOM and mineral will promote sorption of OPs thus produce complementary effect.Our previous systematic studies have revealed that the interfacial sorption behavior of ionizable hydrophobic pentachlorophenol(PCP)and polar hydrophilic butachlor in soil were controlled by both SOM and minerals.In soils with high total organic carbon(TOC)and the ratio of clay content to TOC content(RCO)values less than 60,minerals were strongly associated with SOM and thus favorable sorption sites within SOM were physically occupied by minerals,thereby induced an inhibition effect of minerals on butachlor sorption.However,in the soil with RCO values higher than 60,highly exposed surface of mineral colloids could directly contribute to and enhance butachlor sorption.Nonetheless,our previous studies were based on only limited samples,and the parameters regarding the interaction between SOM and minerals were not taken into account in the formula for deducing the equilibirium sorption coefficient for minerals during sorption of OPs in soil.For an improved understanding of the dual functions and contributions of minerals on OPs sorption in soil,36 soils with RCO values evenly distributed were selected from 609 soil samples.Meanwhile,two representative pollutants,the ionizable pentachlorophenol(PCP)and the non-ionizable phenanthrene(PHE)that were one of the typical chlorophenols(CPs)and polycyclic aromatic hydrocarbons(PAHs)respectively,were selected as model OPs.We explored PCP and PHE sorption behavior in natural soils and representative humic acid(HA)sample,respectively,and built a calculation model to quantify the net contribution of soil minerals in OPs sorption by soil.The main results are as follows:All sorption isotherms were better fitted with the Freundlich model than Linear model.The Kd is an index to represent the affinity between sorbates and sorbents,it approached relative unity in the upper Ce range for PCP and PHE sorption.The Kd of PCP were smaller than that of PHE because the hydrophobicity of PHE was higher than that of PCP.Regardless of PCP or PHE,the Kd was notably positively related to the TOC and total nitrogen(TN)while negatively related to RCO value.Soils with relative lower RCO values and higher TOC contents had relative higher Kd values.The mechanism of sorption would mainly consist of sorption on external mineral surfaces and dissolution in internal voids of SOM.When the RCO rises up to a critical value 60,the Kd drops rapidly and remains stable later on within a very low range,likely due to that excessive minerals would block the sorption sites within SOM and thereby weaken the partition process in SOM.In addition,for PCP sorption,hydrophobic interaction might be responsible for high sorption rate at low pH,while increasing electrostatic repulsion and weakened hydrophobic interaction between PCP and negatively charged mineral surfaces would result in low adsorption at high pH.The sorption coefficients Kd-HA and Koc-HA of HA sample were calculated by equation Kd_HA = qe/Ce and Koc-HA = Kd-HA/foc-HA.These two coefficients declined rapidly at low Ce values and approached relative unity in the upper Ce range.The Kd-HA and Koc-HA were 963.8 mg L-1 and 1933.8 L kg-1 for PCP,and 4407 mg L-1 and 8842.2 L kg-1 for PHE,respectively.To increase the reasonability of our model,the Koc-HA was calculated on the basis of average of results in our study and the Koc-HA data reported by previously comparable studies,with the values of 2275 L kg-1 for PCP and 7995 L kg-1 for PHE,respectively.Basing on 164 data from this and other previously comparable studies,for sorption of three typical representative of alarming OPs classes(including hydrophobic but ionizable PCP,hydrophobic and non-ionizable PHE,and polar hydrophilic butachlor)by 66 natural soils with various degrees of organo-mineral aggregation,the sorption model we developed previously was optimized and improved for quantifying the direct positive contribution and indirect negative contribution of minerals in soils.The Kd of bulk soil was assumed to be the sum of contributions from mineral(Kd-min)and SOM(Kd-oc)separately,as well as contribution from interaction between SOM and minerals(Kd-min×Kd-oc)associatedly.We calculated the mineral-water partition coefficient(Kd-min)under two conditions,that is,Kd>Kd-oc and Kd<Kd-oc.Contribution rate(CR,%)of minerals was calculated from%CR = Kd-min/Kd×100.The results indicated that the contribution of minerals showed consistent changes in the sorption of three different types of pollutants(ionizable hydrophobic PCP,non-ionizable hydrophobically PHE and polar hydrophilic butachlor)in soils.And a threshold of RCO was purposed,that is 60,to be used as a critical value for predicting the dual function of minerals on OPs sorption in soil through their association with SOM,where their net contribution will change from positive to negative,or vice versa.This study provided a simple and convenient method for predicting the dual contribution of soil minerals to the adsorption and retention of OPs in soil.It also laid the foundation for the comprehensive assessment of the migration,transformation,degradation and bioavailability of different OPs in different soils.