Effect of Soil Fulvic and Humic Acids on Pb Binding to the Goethite

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Effect of soil fulvic and humic acid on Pb binding to the goethite/solution interface: Ligand Charge Distribution modeling and speciation distribution of Pb Juan Xiong, Liping Weng, Luuk Koopal, Mingxia Wang, Zhihua Shi, Li-Rong Zheng, and WenFeng Tan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05412 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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Environmental Science & Technology 1 / 29

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Effect of soil fulvic and humic acid on Pb binding to the goethite/solution interface:

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Ligand Charge Distribution modeling and speciation distribution of Pb

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Juan Xiong†, Liping Weng ‡, Luuk K. Koopal†,#, Mingxia Wang†, Zhihua Shi†, Lirong Zheng⊥,

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Wenfeng Tan†,*.

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† College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, P.R.

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China.

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‡ Department of Soil Quality, Wageningen University, P.O. Box 8005, 6700 EC, Wageningen, The

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Netherlands.

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# Physical Chemistry and Soft Matter, Wageningen University and research, P.O. Box 8038, 6703 HB,

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Wageningen, The Netherlands.

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⊥ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

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Sciences, Beijing 100039, P.R. China

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*

Corresponding author: Tel: +86-27-87287508; E-mail: [email protected] (W. F. Tan)

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ACS Paragon Plus Environment

Environmental Science & Technology

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Abstract

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The effect of adsorbed soil fulvic (JGFA) and humic acid (JGHA) on Pb binding to goethite was

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studied with the Ligand Charge Distribution (LCD) model and the X-ray absorption fine structure

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(XAFS) spectroscopy analysis. In LCD model, the adsorbed small JGFA particles were evenly located

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in the Stern layer, but the large JGHA particles were distributed over the Stern layer and the diffuse

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layer which mainly depended on the JGHA diameter and concentrations. Specific interactions of HS

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with goethite were modeled by inner-sphere complexes between –FeOH2+0.5 of goethite and COO of

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HS and by Pb-bridges between surface sites and COO- groups of HS. At low Pb levels, nearly 100% of

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Pb was bound as Pb-bridges for both JGFA and JGHA. At high Pb levels and low HS loading,

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Pb-goethite almost dominated over the entire studied pH range; but at high HS loading, the primary

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species was goethite-HS-Pb at acidic pH and goethite-Pb at alkaline pH. Compared with JGFA there

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was a constant contribution of Pb-bridges about 10% for JGHA. The Linear Combination Fit of

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EXAFS, using Pb-HS and Pb-goethite as references, indicated that with increased HS loading more Pb

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was bound to adsorbed HS and less to goethite, which supported the LCD calculations.

-

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Introduction

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Humic substances (HS) and iron-(hydr)oxides are important active soil colloids1. They govern the

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speciation distribution of metal ions (Men+) and this speciation determines the mobility, bioavailability

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and toxicity of metal ions in natural environment. To gain insight in the characteristics of Men+

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adsorption to HS and iron-(hydr)oxides, many experimental studies have been made on Men+ binding

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to HS2-4 and iron-(hydr)oxides5, 6. For these binary systems, respectively, the NICA-Donnan7, 8 or

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WHAM-model VI/VII9, 10 and CD-MUSIC-Electrical Double Layer (EDL) model11, 12 have been used

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successfully to describe the binding behavior with intrinsic parameters. However, in soils both HS and

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iron-(hydr)oxides are simultaneously present and they interact strongly13-17. These interactions not only

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alter the protonation of both iron-(hydr)oxides and HS13, they also affect the amount of Men+ bound and

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the distribution of bound species18, 19. Therefore, in ternary systems of Me/iron-(hydr)oxides/HS the

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Men+ binding will deviate from that in the corresponding binary systems.

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A common way to describe the Men+ binding in ternary system is the Linear Additivity (LA) model15,

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19-23

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model and experiment indicate the effect of the interactions between iron-(hydr)oxide and HS on the

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Men+ binding. Two recently proposed alternatives for the LA modeling that take into account the

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interactions between HS and iron-(hydr)oxide are the natural organic matter/charge distribution

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(NOM-CD) model24, 25 and the more sophisticated Ligand Charge Distribution (LCD) model14, 18, 26.

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The NOM-CD model considers HS adsorption in the compact part of EDL only and has been applied to

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both anion and cation adsorption to the goethite-HS system21, 24, 25. The LCD model allows a HS

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distribution over both the compact and diffuse part of the EDL; for humic acid (HA) adsorption this is a

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more realistic HS configuration than a distribution over the compact part only.

which is the weighted sum of Men+ binding to the two binary systems. The deviations between



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The LCD model is based on the NICA-Donnan and CD-MUSIC-EDL model and uses the parameters

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of these two models obtained from the binary systems in combination with some new parameters that

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characterize the Me/iron-(hydr)oxides/HS interaction and the location of HS in the EDL18. Weng et al.

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have shown that the LCD model could quantitatively describe both cation18 and anion binding27, 28 to

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goethite-HS complex. The results of goethite/HS with anions indicated that the HS acidic groups

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competed with anions for the surface sites and strongly modified the electrostatic potentials at

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goethite/solution interface to affect anion binding27, 28. In a ternary system with cations LCD model has

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been applied to describe the adsorption of Men+ to goethite in the presence of FA only18. The results

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indicated that the formation of (i) cation complexes with the goethite surface sites, (ii) Me-bridges

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between goethite and FA and (iii) cation complexes with acidic groups of adsorbed FA 18.

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Both the spatial distribution of HS at the goethite/solution interface and the potential profile in EDL

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will depend on the size of adsorbed HS. In our previous study the effect of JGFA and JGHA on Pb

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binding to goethite was investigated by comparing the experimental results with LA modeling and the

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role of particle size was emphasized19. The results showed that HS promoted Pb binding to the

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goethite-HS complex strongly, especially at low pH and low Pb concentrations. Analysis of the results

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indicated that Pb-bridges between goethite and HS sites played a role, but a quantitative estimation of

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Pb-bridges couldn’t be made. Furthermore, it was argued that with some conformational change the

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JGFA charge could be accommodated in Stern layer. However, the volume of JGHA was such that the

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charge of JGHA should be positioned in both Stern layer and diffuse layer. Moreover, the difference in

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size between FA and HA should affect the charge and mass of adsorbed HS. Therefore, cation binding

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in ternary systems containing FA or HA should, in principle, be different.

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In binary systems information on Men+ binding at a molecular level has been obtained with (Extended)


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X-ray absorption fine structure (XAFS) spectroscopy analysis2, 29. Also (E)XAFS spectra of ternary

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systems can be obtained and ‘Linear Combination Fitting’ (LCF) of corresponding binary systems

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spectra to the spectrum of ternary system has been used to obtain quantitative information on the

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Me-HS and Me-mineral interactions in ternary system30, 31. However, due to the low Pb binding

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adsorption capacity of goethite and the Pb-EXAFS measurement with fluorescence mode under the

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high fluorescence background of Fe (goethite), no XAFS literature is available on Pb binding

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mechanisms in the Pb/goethite/HS system.

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The purpose of this study is to investigate quantitatively the effects of JGFA and JGHA on Pb binding

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to goethite-HS complexes. To this aim LCD modeling is combined with EXAFS results. (1) The LCD

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model with the parameters taken from the binary systems was fitted to the experimental data described

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in19 to obtain the HS-goethite interaction parameters and the bound Pb speciation distribution of

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Pb/HS/goethite system. (2) The EXAFS spectra of the binary and ternary systems were used to

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calculate the relative contributions of the HS-Pb and goethite-Pb complexes and the results were

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compared with the bound Pb speciation distribution of LCD calculations. The results of analysis

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provide quantitative insight in the differences between JGFA and JGHA on their effects on Pb binding

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to the goethite-HS complex in particular, and to Men+ binding to HS bound to metal-(hydr)-oxides in

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general.

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Experimental data and Methods

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Binary systems. The experimental Pb binding data of goethite in this study were taken from Xiong et

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al.19. Goethite had a BET-N2 SSA of 85m2/g and a PZC of 9.1. The proton and Pb binding to JGFA and

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JGHA have been described in2, 32. The average particle mass of JGFA was 2.6kDa19 that of JGHA

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38kDa33, based on these data and assuming for hydrated HS a mass density of 1250kg/m3 the



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calculated particle diameters were, respectively, about 1.5nm and 3.6nm19.

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Ternary systems The Pb binding experiments of the ternary systems have been described in detail in

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the previous paper19. Briefly, 5ml Pb(NO3)2 and 5ml HS solution were added simultaneously to 10ml

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10g/L or 2g/L goethite suspensions. The initial Pb concentrations were 0.005mmol/L and 1mmol/L for

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JGFA, 0.005mmol/L and 0.5mmol/L for JGHA. The final total HS concentrations were 75 and

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450mg/L for JGFA, 150 and 450mg/L for JGHA. Small volumes of HNO3 or KOH solutions were used

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to adjust the suspension pH to desired value in the range of 3.0 to 11.0. The prepared suspensions were

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shaken at 25oC for 72h, and subsequently centrifuged at 16261g for 30min. The dissolved HS and total

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Pb concentration in the supernatant were determined by, respectively, a TOC meter and atomic

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absorption spectroscopy.

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XAFS Spectroscopy The goethite-HS-Pb complex was prepared at pH 5.0 and 0.1mol/L KNO3 and

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high Pb concentrations (1mmol/L for JGFA and 0.5mmol/L for JGHA). The process of sample

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preparation was the same as that of batch adsorption experiments. The XAFS spectra of all samples

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were collected with the fluorescence mode and recorded at Pb L3-edge (E=13035eV) at room

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temperature on the 1W1B beamline at Beijing Synchrotron Radiation Facility. Detailed descriptions of

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the XAFS measurements are presented in Supporting Information (SI). The raw data analysis was

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performed using the software Athena0.8.056 following procedures described elsewhere34. The ternary

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system spectra were analyzed using LCF of the two reference spectra (HS/Pb, goethite/Pb) to the

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goethite/HS/Pb spectra

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goethite/HS/Pb systems. The data fitting ranged from 2.0 to 7.0 or 7.5Å−1, depending on the data

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quality31.

30, 31

to obtain the information on the HS/Pb and goethite/Pb interactions in the


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Revised LCD Model and its parameters

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Outline. The LCD model18, 26considers the interaction between HS and metal-(hydr)oxides in the

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presence of a background electrolyte and specifically adsorbing ions. To specify the ternary

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goethite/HS/Pb system the NICA and CD-MUSIC model parameters, obtained by investigating the

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HS/Pb and goethite/Pb binary systems, were used, but some new parameters have to be introduced as

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well. The additional parameters are specifying the interaction between HSads and goethite and the HSads

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distribution over the inner and outer Stern layer and diffuse layer at the goethite-HS/solution interface.

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The experimentally measured HSads values are used as model input to simplify the LCD calculations.

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The model is implemented with the software ORCHESTRA35 in which the model equations can be

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easily extendible defined by the users and the calculation is carried out numerically in an iterative way.

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Reactions in the ternary system Traditionally ion binding to dissolved HS is considered to occur by

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complexation

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(RCOH/RCO-). The NICA-Donnan model for proton binding is calibrated with proton binding data32.

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By combination with the EXAFS results the NICA-Donnan model for Pb binding was further

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calibrated with Pb binding isotherms2. The obtained material-specific model parameters of proton and

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Pb binding to HS are reproduced in Table S3. In the LCD model, these parameter values were used to

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characterize the specific interaction of protons and Pb2+ to the functional groups of both dissolved HS

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and adsorbed HS.

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The ion binding to goethite was modeled with the CD-MUSIC-EDL model, but the site binding part of

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model used was simpler than that used in the previous study. The simplifications have been made to

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reduce the number of parameters, which was especially relevant for the LCD calculations. Based on

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spectroscopic results11 four different surface complex with Pb were considered. The detailed

with

the

heterogeneous

carboxylic

(RCOOH/RCOO-)


and

phenolic

groups


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description of the model and fitting was present in SI. The fitted results were depicted in Figure S2 and

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the model parameters were collected in Table S1, S2. The four different types of Pb-complexes will be

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indicated as ‘goethite-Pb’. In the LCD model, the specific interaction of proton and Pb to the surface

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sites of goethite was characterized by the same parameter values.

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Between adsorbed HS and goethite two types of specific interaction may occur: (1) binding of RCOO-

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and/or RO- of HS adsorbed in inner Stern layer to –FeOH2+0.5 sites of goethite leading to, respectively,

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-FeOOCR-0.5 and –FeOR-0.5 (noted as goethite-HS)26, 36 and (2) binding of RCOO- and RO- of HS

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located in the inner Stern layer to (-FeOH)2-Pb+1 leading to bridging complexes (-FeOH)2-Pb-OOCR0

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and (-FeOH)2-Pb-OR0 (noted as goethite-Pb-HS). The analysis of the adsorption of HS to goethite with

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LCD modeling

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complexes are dominant; therefore, the –FeOR-0.5 complexes are neglected with the present

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calculations. Preliminary LCD calculations regarding Pb-bridges indicated that the fitting results hardly

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improved by including (-FeOH)2-Pb-OR0. Lu et al37 have demonstrated preferential involvement of

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RCOO- of HS in Pb binding and this suggests that (-FeOH)2-Pb-OOCR0 will be more likely than

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(-FeOH)2-Pb-OR0. To simplify the calculations (-FeOH)2-Pb-OR0 are neglected in the further

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calculations.

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HSads distribution and conformation in the EDL. Adsorbed HS protrudes in solution and as a

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consequence the EDL is affected. The extent of the effect is determined by the fractions HS present in

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the inner- and outer-Stern layer and the diffuse layer. In general a fraction fHS1 is present in the inner

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Stern layer and the charges of this fraction are placed at 0-plane and/or at 1-plane depending on their

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complexation state. A fraction fHS2 is present in the outer Stern layer; the net charge of this fraction is

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placed at 2-plane. The remaining fraction fHS3 is extending in the ‘diffuse’ layer and perturbs the diffuse

14, 18

and a study of FTIR spectra36 lead to the conclusion that the –FeOOCR-0.5


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potential distribution. In order to accommodate the HS charge of fraction fHS3 an additional third plane

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is placed in EDL at distance ∆ from the 2-plane. The 3-plane is now the head-end of the unperturbed

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diffuse layer. All charges of the EDL that have not yet been compensated should thus be placed on the

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3-plane. The ‘diffuse’ charge density in between 2-plane and 3-plane is estimated by using ∆ and the

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local ‘diffuse’ ion concentration as calculated with the Boltzmann equation with the potential of

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3-plane. In accordance with previous work14, a distance ∆=1nm has been assumed. The net charge

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density of 3-plane is compensated in the unperturbed diffuse layer extending beyond the 3-plane.

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To estimate the distribution of HSads over the three HS fractions present in EDL it should be taken into

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account that due to HS adsorption some flattening of the HS particles occurs. A similar conclusion was

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reached by Hiemstra et al.24 on the basis of the strong competition effects between HA and phosphate.

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Based on our previous study19 and in agreement with18, it is assumed that adsorbed JGFA is only

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present in Stern layer, i.e., fFA3=0. As in18 an equal distribution of JGFA over the inner and outer Stern

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layer is assumed, i.e., fFA1=fFA2=0.5. This distribution implies a flattening of about 50% because the

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JGFA particle diameter is about 1.5nm and the Stern layer thickness is about 0.8nm. For the present

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goethite (85m2/g) the maximum possible adsorbed amount of HS (hydrated volume 1250kg/m3) that

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can be accommodated in a Stern layer is about 85mg-HS/g-goethite. This value corresponds well with

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the observed maximum adsorption of JGFA (78mg/g) at low pH and high Pb levels19. Thus for JGFA

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the flattening approximation is reasonable and up to high adsorbed amounts all FA can still be

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accommodated in Stern layer.

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In the case of JGHA and low loading flattening may occur but only about 85mg/g JGHA can be

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accommodated in Stern layer. When JGHA particles don’t flatten or stretch (layer thickness 3.6nm) the

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calculated maximum adsorbed amount is about 385mg/g, which corresponds reasonably well with the



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experimentally observed maximum JGHA adsorption at low pH (414mg/g)19. Therefore, flattening of

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JGHA will not persist at large adsorbed amounts. When it’s assumed that at low adsorbed amounts HA

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can flatten to about the total Stern layer thickness (about 20% of JGHA diameter in solution) then at

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most 85mg-JGHA/g-goethite can be accommodated in Stern layer. Such a strong flattening of the HA

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particles has also been assumed by Hiemstra et al.24, who obtained good results by locating even larger

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HA particles in the compact part of EDL at low adsorbed amount. Although at most 85mg/g JGHA can

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be accommodated in entire Stern layer, at already somewhat lower adsorbed amounts HAads will be

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likely also present in the diffuse part of the EDL. For JGHA it will therefore be assumed that up to a

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volume fraction of HA in Stern layer of 0.7 (adsorbed amount of 60mg/g) all HA is present in Stern

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layer with equal fractions for the inner and outer Stern layer, i.e., fHA1= fHA2=0.5 and fHA3=0

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(Qads≤60mg/g). For higher adsorbed amounts additional JGHA is also present in diffuse layer. For

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Qads>60mg/g the amounts of JGHA in both diffuse layer and Stern layer gradually increase; by keeping

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fHA1=fHA2 and assuming an increase of adsorbed JGHA in Stern layer (up to 85mg/g) and in the diffuse

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layer, the fractions fHA1, fHA2 and fHA3 can be calculated using the following set of equations:

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f HA1 = f HA2 = ad − HA +

fHA3 = 1− 2 fHA1

bs − goe

Qads

+

c (eq-1) Qads (eq-2)

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where the adsorbed amounts (Qads) are in mg/g, ad-HA is a constant related to the HA diameter; bs-goe is a

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constant related to the SSA of goethite; c is a constant related to the interaction between goethite and

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HA and can be calculated from ad-HA and bs-goe.

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Speciation calculation with LCD-NICA-EDL and LCD-CD-MUSIC-EDL. As the ion binding

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reactions occur with the heterogeneous RCOO- and RO- the LCD-NICA-EDL part of model should be

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used to calculate the amounts of these HS-complexes; the fact that the goethite-HS complex is involved


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implies that the EDL potentials of goethite/solution interface have to be used. Therefore, for each HS

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fraction, the LCD-NICA equation has to be combined with an appropriate electrostatic Boltzmann

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factor that contains the characteristic electrostatic potential(s) of that fraction. With the functional

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groups of fraction fHS1 ‘ion’ binding may occur with H+, Pb2+, -FeOH-0.5 and (-FeOH)2-Pb+1. The

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binding reactions and parameters for H+ and Pb2+ were the same as for HS in solution and the charges

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were placed at 1-plane.With respect to the reactions of HS functional groups with the surface sites only

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-FeOOCR-0.5 and (-FeOH)2-Pb-OOCR0 complexes were considered. The NICA affinity parameters and

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charge distribution of -FeOOCR-0.5 were adopted from Weng et al.14, 18. The non-ideality parameter

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(nFe,1) of this reaction was assumed to be the same as for protons (nH,1), leading to a 1:1 stoichiometry.

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The NICA parameters of (-FeOH)2-Pb-OOCR0 have been selected in accordance with the binary

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system parameters, i.e. the nPb,1 was assumed to be equal to that for Pb binding to RCOO- of HS and the

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charge of RCOO- was placed at 1-plane. The charge distribution of Pb2+ was assumed to be same as

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that of (-FeOH)2-Pb+1, ∆z0=1.2 and ∆z1=0.8. The logK of -(FeOH)2-Pb-OOCR0 was optimized by

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fitting the model results to the experimental data.

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With the HS fractions fHS2 and fHS3 ion binding may occur with H+ and Pb2+. The states of these HS

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fractions differ only from that of HS in solution due to the different electrostatic potentials that the

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RCOO- and RO- experience. For fraction fHS2 all charges were placed at 2-plane and for fHS3 at 3-plane.

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The total Pb binding to the three HSads fractions is denoted as goethite-HS-Pb.

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The first LCD-NICA-EDL calculation starts with an estimated EDL potential profile and this

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calculation results have to be consistent with the LCD-CD-MUSIC-EDL calculations. Therefore, the

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speciation calculated with LCD-NICA-EDL was entered in the LCD-CD-MUSIC-EDL part and the

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surface site speciation and electrostatic potential distribution were recalculated. The resulting potential



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distribution was introduced in LCD-NICA-EDL part again and the calculation was repeated. This

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process was repeated till the calculations converge and a self-consistent site speciation and potential

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distribution was obtained. The final parameters are collected in Table 1. The obtained state of the

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goethite-HSads system provides the equilibrium speciation of bound Pb.

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Result and Discussion

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LCD modeling

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The LCD modeling results for Pb binding to goethite-HS complexes are compared with the

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experimental results in Figure 1 and 2 for, respectively, JGFA and JGHA. In the figures two different

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LCD results are presented: (1) dotted curves, only specific HS binding through complexation of

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RCOO- with the goethite surface groups (goethite-HS), and (2) solid curves, specific HS binding

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through RCOO- complexation with the goethite surface groups and through formation of Pb-bridge

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(goethite-Pb-HS). In both cases Pb was allowed to bind to goethite surface sites and to the functional

243

groups of adsorbed HS.

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As seen in Figure 1, the LCD results for JGFA based on only goethite-HS interaction underestimated

245

the Pb binding at low pH and 0.005mmol/L Pb (dotted lines in Figure 1a, 1b), but agreed well with the

246

experimental Pb binding to the goethite-JGFA complex at 1mmol/L Pb (dotted lines in Figure 1c, 1d).

247

At 0.005mmol/L Pb the fit between LCD calculations and Pb binding experiments could be improved

248

by including, besides goethite-HS interaction, also Pb-bridges (solid lines in Figure 1a, 1b). The fact

249

that at 0.005mmol/L Pb Pb-bridges improved the results, whereas they were not required at 1mmol/L

250

Pb, suggesting that there is a small fraction of RCOO- of JGFA that has a high affinity for Pb-bridges.

251

At 1mmol/L Pb, this species is still present, but due to its small capacity it hardly plays a role as

252

compared to the goethite-Pb and goethite-HS-Pb that have a much larger adsorption capacity. The fact


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that the LCD calculations overestimated the bridges contribution at 1mmol/L Pb might be caused by a

254

somewhat too high affinity for the formation of (-FeOH)2-Pb-OOCR0 and/or of the formation of

255

(-FeOH)2-Pb+1. A similar behavior was observed for Cu binding to the goethite/SFA system18,

256

Cu-bridges was less important at high Cu concentrations than at low Cu concentrations.

257

The LCD model results for JGHA provided, in general, a good description of Pb binding when both

258

surface complexation and Pb-bridges were included. When only surface complexation with JGHA was

259

allowed, the LCD model always underestimated the Pb binding to goethite-JGHA complex. The latter

260

couldn’t be improved by adjusting the fraction of adsorbed JGHA in the Stern layer. An accurate

261

description of Pb binding to the goethite-JGHA complex at 0.005mmol/L Pb was observed, but at

262

0.5mmol/L Pb the Pb binding was slightly underestimated, except for pH7.5. At 450mg/L JGHA and 0.5mmol/L Pb (Figure

305

2d), the trends for goethite-Pb and goethite-JGHA-Pb were similar to those at 150mg/L JGHA and

306

0.5mmol/L Pb, but the relative contribution of goethite-JGHA-Pb was higher, especially at about pH

Effect of Soil Fulvic and Humic Acids on Pb Binding to the Goethite

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