Effect of Organic Matter on Sorption of Zn on Soil: Elucidation by Wien

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Effect of Organic Matter on Sorption of Zn on Soil: Elucidation by Wien Effect Measurements and EXAFS Spectroscopy Tingting Fan, Yu-Jun Wang, Cheng-Bao Li, Jian-Zhou He, Juan Gao, Dongmei Zhou, Shmulik P. Friedman, and Donald L. Sparks Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05281 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on February 19, 2016

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Environmental Science & Technology

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Effect of Organic Matter on Sorption of Zn on Soil: Elucidation by Wien

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Effect Measurements and EXAFS Spectroscopy

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Ting-Ting Fan1, 2, Yu-Jun Wang1*, Cheng-Bao Li1, Jian-Zhou He1, 2, Juan Gao1, Dong-Mei

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Zhou1*, Shmulik P. Friedman3, and Donald Sparks4

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1

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the Chinese Academy of Sciences, Nanjing 210008, China

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2

University of Chinese Academy of Sciences, Beijing 100049, China

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3

Institute of Soil, Water, and Environmental Sciences, Agricultural Research Organization,

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Volcani Center, Bet Dagan 50250, Israel

Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science,

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4

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and Soil Sciences, University of Delaware, Newark, Delaware 19717-1303, USA

Environmental Soil Chemistry Group, Delaware Environmental Institute and Dept of Plant

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*Corresponding

author,

Tel:

0086-25-86881182,

Fax:

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[email protected] (Yu-Jun Wang), [email protected] (Dong-Mei Zhou)

1

ACS Paragon Plus Environment

0086-25-86881000,

E-mail:


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Abstract

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Soil organic matter (SOM) is the major factor affecting sequestration of heavy metals in

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soil. The mean free binding energy and the mean free adsorption energy and speciation of Zn

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in soil, as affected by SOM, were determined by employing Wien effect measurements. The

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presence of SOM markedly decreased the Zn binding energy in soils in the order: Top (5.86

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kJ mol-1) < Bottom (8.66 kJ mol-1) < Top OM-free (9.44 kJ mol-1) ≈ Bottom OM-free (9.50 kJ

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mol-1). The SOM also significantly decreased the adsorption energy of Zn on black soil

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particles by reducing non-specific adsorption of Zn on their surfaces. The speciation of Zn in

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soils was elucidated by extended X-ray absorption fine structure (EXAFS) spectroscopy and micro-focus

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X-ray fluorescence (µ-XRF). The results obtained by linear combination fitting (LCF) of EXAFS

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spectra revealed that the main forms of Zn in soil were outer-sphere Zn, Zn-illite, Zn-kaolinite,

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and HA-Zn. As the SOM content increased, the proportion of HA-Zn among the total

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immobilized Zn increased and the proportion of non-specific adsorbed Zn decreased. The

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present results implied that SOM is an important controlling factor for the environmental

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behavior of Zn in soils.

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Key words: Zn, organic matter, adsorption energy, binding energy, EXAFS, µ-XRF

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Introduction

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Zinc (Zn) is a key element for plant growth and human health; it is essential for the

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normal activity of DNA polymerase and protein synthesis, and it plays a vital role in the

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healthy development of many life forms.1, 2 Excessive amounts of Zn, however, can be toxic,

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not only to plants and animals but also to humans.3 Plants obtain their Zn from the soil

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solution mainly in the form of ions or chelates.2 Adsorption controls the sequestration and

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mobility of heavy metals in soils, and it is well known that variations in soil properties such as

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pH, cation exchange capacity (CEC), texture, and soil organic matter (SOM) content

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significantly influence the adsorption of heavy metals.

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The term SOM refers to all natural and biologically- derived organic materials found in

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the soil,4 which includes organic matter associated with soil particles and soluble or dissolved

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organic matter (DOM). SOM has a high specific surface area (approximately 800-900 m2 g-1)

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and a CEC that ranges from 150 to 300 cmol kg-1.5 The SOM includes some functional groups

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such as carboxylates and phenolics, therefore, it can be complexed with metals, which affects

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their retention and mobility in soils. 5, 6 DOM enhances the solubility of metals and organic

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matter associated with soil particles adsorbs metals, thus reduces their solubility and mobility.

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Sauve et al. demonstrated that higher soil organic matter content was associated with also

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increased DOM, and thus promoted the formation of organo-Pb complexes, which enhanced

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Pb solubility. 7 Mesquita and Carranca found that the presence of DOM decreased adsorption

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of Cu and Zn to soil particles.8 We also found that some low-molecular-weight organic acids,

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such as malic, succinic, citric, acetic, oxalic, and tartaric acids, significantly decreased the

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adsorbed amount of Cu(II) on hydroxyapatite (HAP) particles in the clay-size fraction.9 3



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Comparison of the effects of various organic acids showed that the maximum quantity of

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Cu(II) adsorbed onto the HAP particles increased exponentially with the cumulative

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formation constants of Cu(II) and the organic acids.9 In addition to experimental results, Weng

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et al. found that DOM could increase the concentration of dissolved metals, although

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metal-DOM complexes comprises only a small fraction of metals sorbed on soils by the

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theoretical calculations with a NICA-Donnan model.10

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Some of SOM is present as a coating or a thin layer on the mineral surfaces,11 and its

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removal changes the sorption of heavy metals in the soils, i.e., the sorption capacity increases

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or decreases. Shuman, using batch adsorption experiments, found that the removal of SOM by

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sodium hypochlorite lowered Zn sorption capacity and decreased Zn binding energy.

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However, Hinz and Selim, who used the thin-disk flow method, reported that the removal of

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OM resulted in a two- to four- fold increase in Zn retention. 13 Trehan and Sekhon, using

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traditional batch experiments, also found that the removal of SOM significantly increased

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sorption of Zn on soil. 14 As indicated above, sorption of Zn is largely controlled by SOM,

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which thereby determines Zn bioavailability in soils. Thus, some contradictory results have

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been published regarding SOM effects on Zn sorption capacity of soil particles, which

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indicates that the effect of SOM on Zn sorption in soils is not well understood.

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In the last decade, the increase with applied electrical field (E), of the electrical

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conductivity (EC) of soil/water/electrolyte systems, termed as the Wien effect, has been

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measured successfully by our research group. The Wien effect method enables evaluation of

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the mean Gibbs free adsorption and binding energies of metal cations on soil particles.15-20 In

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a previous Wien effect study we found that the removal of SOM slightly decreased the 4


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binding energies, which represent the whole spectrum of adsorption energies, of K+, Ca2+, and

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Cd2+ on a paddy soil, but that the adsorption energies, which represent the fraction of low

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adsorption energies of these metals on this soil, significantly increased.21 In contrast to these

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findings, the removal of SOM from a black soil, richer in organic matter than the paddy soil,

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increased the binding energies of Cd2+, Cu2+, Pb2+, and Ca2+.22

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How does SOM affect the sorption of metals on soils? In the present study, the effect of

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SOM (both DOM and OM associated with the solid particles) on the interaction between Zn

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and clay-fraction particles was elucidated via the Wien effect method. This method provides

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quantitative information on the distribution (spectrum) of adsorption energies and direct

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insights on the effect of SOM on the adsorption mechanism. Complementary extended X-ray

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absorption fine structure (EXAFS) spectroscopy and micro-focus X-ray fluorescence (µ-XRF)

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were employed to determine the speciation and distribution of Zn in soils. The main

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objectives were to explore the effects of SOM on the molecular mechanisms of Zn sorption on

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soil particles and on the speciation of Zn immobilized in soil, and thereby to enhance and

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deepen the characterization, quantification and understanding of the environmental behavior

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of Zn in soils.

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MATERIALS AND METHODS

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Soil Samples

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Black soil samples from Hailun, Heilongjiang Province, China were collected from depths

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of 0 – 20 cm (designated as top soil) and 100 – 120 cm (designated as bottom soil). The basic

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properties of the sampled soils were listed in Table S1. After the samples were dried, ground, 5



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and passed through a 60-mesh sieve, the clay-fraction particles, less than 2 µm in diameter,

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were separated by sedimentation. Organic matter (OM) was removed from clay fractions of

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the top and bottom soil samples by adding 6% H2O2,23 these samples labeled as “Top OM-free”

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and “Bottom OM-free” (the clay-fraction particles without removal of OM labeled as “Top”

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and “Bottom”). The mineral composition of the clay-fraction particles with and without

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removal of OM was determined by X-ray diffraction (XRD) analysis and the results are listed

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in Table S2. The top black soil-fraction particles contained mainly kaolinite and illite; the

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bottom black soil-fraction particles were richer in 2:1 silicate minerals than the top soil (Table

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S2). The removal of the OM did not alter the mineral composition of the soil clay fractions.

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The positive and negative charge densities of the clay fraction of the soil samples at various

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pH values were determined by the modified Schofield method and were presented in Figure

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S1. The samples carried almost no positive charges, which met the requirements of the

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applied Wien effect method.15 The OM removal increased the negative charge densities on the

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particles at near-neutral pH values, but did not significantly affect the negative charge

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densities under the experimental pH conditions, i.e., pH 5 (Table 1).

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Preparations of Homoionic Soil Samples and Suspensions

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25 mL of 0.5 mol L-1 ZnCl2 solution was added into a 0.5 g sample of the original and

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OM-removed clay fraction particles; the resultant mixture was shaken for 5 h at 25 °C and

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centrifuged, then the supernatants were discarded. This procedure was repeated three times.

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Subsequently, deionized (DI) water was added into the tubes to thoroughly wash the clay

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paste separated from centrifugation until Cl- couldn’t be detected in the discarded

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supernatants. Finally, the samples (hereafter referred to as Zn-saturated samples) were dried 6


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and ground to prepare the suspensions for the Wien effect measurements and the

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powder-based tablets for the EXAFS and µ-XRF measurements. The Zn content of

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Zn-saturated samples was shown in Figure S2.

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0.25 g Zn-saturated samples and 25 mL DI water were added into 50-mL plastic

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centrifuge tubes to achieve a solid concentration of 10 g L-1. The centrifuge tubes were sealed

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and mixed followed by their ultrasonically dispersing for 45 min and oscillating for 1 h.

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Hereafter, the suspensions were shaken for 1 h daily for 7 to 10 days to achieve equilibration

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of ion reactions. During the equilibration period, the ECs of suspensions were regularly

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monitored. When little or no change of ECs was observed, equilibration was considered

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acceptable and the suspensions were ready to be used to conduct the Wien effect

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measurements. All suspensions were prepared in duplicate.

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Wien Effect Measurements

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The electrical conductivity under strong electrical fields was measured with the SHP-2

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(short high-voltage pulse) apparatus. The structure of the SHP-2 apparatus and the measuring

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procedure were outlined in detail in previous publications.15-17, 21, 24 Before measuring the

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Wien effect with the apparatus, the weak-field EC of the sample was determined with a

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regular conductivity meter (DDS-310, Shanghai Kangyi instrument co.) at a constant room

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temperature of 25 ºC to ensure that the resistance of the test suspension was within the

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measurement range of 200 Ω to 20 kΩ. The strong-field ECs of the suspensions were

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measured by applying a voltage drop that increased from 1.0 kV up to the occurrence of

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sparking (dielectric breakdown) in the suspensions, which were held in a thermostatic

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chamber at 25ºC. The electrode spacing was kept constant at 1 mm. After a phase of 7



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increasing electrical field strength, the measurements were repeated in reverse, with

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decreasing field strength, in order to eliminate possible effects of long-term heating and other

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irreversible phenomena.

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Evaluation of Mean Gibbs Free Binding and Adsorption Energies

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An analogy between ion activity and electrical conductivity was assumed for evaluating

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the mean Gibbs free binding and adsorption energies. The measured pH values of the tested

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suspensions and the charge density curves of the soil particles (Figure S1) were used to

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calculate the CEC at the specific suspension pH. The measured weak-field electrical

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conductivity (EC0) was used to calculate the mean Gibbs free binding energy according to Li

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et al.15:

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∆Gbi = RT ln

2CEC × C p × λ

[1]

EC0

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in which R is the universal gas constant (8.315 J mol–1 K–1), T is the thermodynamic

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temperature (K), CEC is the negative charge density (mol kg–1), Cp is the solid concentration

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of the suspension (g L-1), and λ is the equivalent conductivity of Zn (52.8 mS cm–1 mol–1 L).

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In accordance with the principle of thermodynamic equilibrium, and with defining states (1)

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and (2) as the ECs under the weak and strong electrical fields, the mean Gibbs free adsorption

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energy was evaluated from15

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∆Gad = RT ln(EC/EC0）

[2]

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in which EC is the strong-field electrical conductivity obtained from the Wien effect

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measurement results, i.e., the EC(E) curves.

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EXAFS and µ-XRF Measurements

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Zn K-edge (9659 eV) EXAFS measurements were conducted at beamline BL14W at the 8


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Shanghai Synchrotron Radiation Facility (SSRF) in fluorescence modes. The spatial

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distributions of elements (Zn, Fe, Mn and K) in the samples were recorded at the beamline

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BL15U at the SSRF. Details of the EXAFS data and µ-XRF data collection procedures and

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data analysis were described in the supporting information (SI) following published

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documentation.25, 26

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RESULTS AND DISCUSSION

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EC-E Curves of Suspensions of Black Soil Particles Saturated with Zn2+

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The repeated (increasing-/decreasing-E phases) EC-E curves of suspensions of

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Zn-saturated soil particles (original and OM-free) were similar, therefore only one of the two

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sets was presented in Figure 1. It was evident that the electrical conductivity of all samples

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increased nonlinearly with electrical field strength. The suspensions of Top samples had

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higher EC than the OM poorer and the OM depleted samples. It was interesting that the EC-E

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curve of Top OM-free samples was almost identical to that of Bottom OM-free samples.

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Though top and bottom black soil particles had differing clay-mineral compositions because

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they were sampled from different horizons, they elicited similar EC-E curves after the OM

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was removed. This indicated that the SOM was the major factor determining the EC-E curves

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of the different soil horizons. The rate of EC increase of the original top black soil particles

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was the lowest, the order of EC increase being Top < Bottom < Top OM-free ≈ Bottom

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OM-free. This indicated that, within the experimentally observed range of field strengths, the

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tested OM-free suspensions released more zinc ions and thereby contributed more to the

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suspension EC than the OM-containing suspensions. Figure 1 also showed that the weak field 9



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(< 15 kV cm-1) conductivities (EC0) of the suspensions increased with increasing OM content

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in the order: OM-free (0.0073–0.0076 mS cm-1) < Bottom (0.0124 mS cm-1) < Top (0.0321

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mS cm-1). The observed order of the EC0 values of the original and OM-free samples was

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consistent with previous findings.21 Soil samples with more OM also were richer in dissolved

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organic matter (DOM) (Table S1). Because the point of zero charge (PZC) of the soil organic

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matter was low, at about pH 3, in the studied higher pH range it was a variable-charge soil

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component.5 The SOM contained a number of oxygen-containing functional groups,27 some

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of which were carboxyls (pKa < 5) and quinones, which dissociated readily to form ions that

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contributed to the soil-suspension EC0. In contrast, the organic matter dissolved in the soil

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solution comprised mainly low-molecular-weight and low-aromatization-state organic

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molecules that could form soluble complexes with metals. These complexes could be ionized

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to form central cations and ligands with electroconducting functional groups, thereby

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increasing the electrical conductivity of soil suspensions.6 Cabaniss demonstrated this

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phenomenon and found that the affinity of Zn(II) and Cd(II) to DOM was in general weaker

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than that of Cu(II), Pb(II), and Ni(II). 28 It was also confirmed by Pandey et al. employing

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anion exchange equilibrium method that the order of stabilities of complexes formed between

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metals and humic acid isolated from a natural soil is Cu (5.28) > Fe (5.03) > Pb (3.66) > Ni

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(3.20) > Co (2.82) > Ca (2.78) > Cd (2.78) > Zn (2.74) > Mn (2.62) > Mg (2.35).29 Bai et al.

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also found that the lower was the aromatization of humic acid, the smaller were its

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coordination number and its complexation constant with Zn or Cd. 30

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Effect of OM on the Mean Free Binding Energy of Zn2+ to Soil Particles

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The mean free binding energies of the soil suspensions were evaluated by using Eq. (1) 10


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with the parameters listed in Table 1. The mean free binding energy of Zn2+ to the soil

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particles decreased with increasing SOM content (Table 1) in the order: Top (5.86 kJ mol-1)

Effect of Organic Matter on Sorption of Zn on Soil: Elucidation by Wien

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