Copper, Nickel and Zinc Speciation in a Biosolid-Amended Soil: pH

Jun 4, 2014 - S.S. Mayakaduwa , Indika Herath , Yong Sik Ok , Dinesh Mohan , Meththika Vithanage. Environmental Science and Pollution Research 2017 ...
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Copper, Nickel and Zinc Speciation in a Biosolid-Amended Soil: pH Adsorption Edge, μ‑XRF and μ‑XANES Investigations Yannick Mamindy-Pajany,† Stéphanie Sayen,† J. Frederick W. Mosselmans,‡ and Emmanuel Guillon*,† †

Institut de Chimie Moléculaire de Reims (ICMR), UMR CNRS 7312, Université de Reims Champagne-Ardenne, Faculté des Sciences, B.P. 1039, Reims 51687 Cedex 2, France ‡ Diamond Light Source, Harwell Science and Innovation Campus, Didcot, OX11 0DE, United Kingdom ABSTRACT: Metal solid phase speciation plays an important role in the control of the long-term stability of metals in biosolid-amended soils. The present work used pH-adsorption edge experiments and synchrotron-based spectroscopy techniques to understand the solid phase speciation of copper, nickel and zinc in a biosolid-amended soil. Comparison of metal adsorption edges on the biosolid-amended soil and the soil sample showed that Cu, Ni, and Zn can be retained by both soil and biosolid components such as amorphous iron phases, organic matter and clay minerals. These data are combined with microscopic results to obtain structural information about the surface complexes formed. Linear combination fitting of K-edge XANES spectra of metal hot-spots indicated consistent differences in metal speciation between metals. While organic matter plays a dominant role in Ni binding in the biosolidamended soil, it was of lesser importance for Cu and Zn. This study suggests that even if the metals can be associated with soil components (clay minerals and organic matter), biosolid application will increase metals retention in the biosolid-amended soil by providing reactive organic matter and iron oxide fractions. Among the studied metals, the long-term mobility of Ni could be affected by organic matter degradation while Cu and Zn are strongly associated with iron oxides.



INTRODUCTION Biosolids are inevitable byproducts of the treatment of municipal or industrial wastewater treatment plants.1 Due to the intensification of water quality requirements and more rigorous environmental laws in Europe, the production of biosolids has sharply increased in recent years.2,3 The main management options include incineration, landfill sites, and application to agricultural lands as soil amendment.4 The use of biosolids in agriculture is widely practiced all over the world since it is the most economical option allowing the recycling of plant nutrients and organic matter.5−7 However, this practice can be potentially harmful to the environment because biosolids contain high concentrations of toxic heavy metals.8,9 Copper, nickel, and zinc are essential micronutrients required for plant nutrition and their addition with biosolids may be beneficial, but they are also potential environmental contaminants and will persist in the soil long-term if added in excess.10,11 The fate of potentially toxic elements following short and long-term application of biosolids is well reported in the literature11−14 but this issue remains strongly controversial within the scientific community. Some published data have shown that the long-term biosolid spreading would result in the release of metals into the soil due to organic matter mineralization; it is the “time bomb hypothesis”.10,15,16 Other authors have suggested that the long-term application of biosolids would present no environmental risk, due to the high adsorption capacity of mineral phases within biosolids; it is the “protection hypothesis”.14,17,18 Consequently, to better predict © 2014 American Chemical Society

the environmental fate and mobility of contaminants, it is critical to study the speciation of heavy metals in biosolidamended soils. This can be achieved by traditional chemical methods such as adsorption/desorption experiments or spectroscopic techniques such as X-ray absorption spectroscopy (XAS) and X-ray fluorescence microscopy.19,20 Synchrotron radiation techniques are the best available techniques for examination of metal speciation and associations in complex environmental media due to their high resolution and to their selectivity in terms of studied element. The published data highlighted the influence of treatment processes on metal solid phase speciation within biosolids. For example, some works have shown that sulfide minerals played a dominant role in Cu and Zn binding in freshly dewatered biosolids while they were of lesser importance in dried biosolids that have been stockpiled.19,21 Other works on oxidized biosolids (e.g., after drying, composting, or liming) have demonstrated that Cu and Zn speciation was mainly controlled by organic matter (especially for Cu) and minerals such as Fe oxides.14,20 These studies also show that the employment of synchrotron-based research methods to examine metal solid phase speciation in biosolids gives useful information to predict metal mobility and bioavailability in biosolid-amended soils. However, such studies Received: Revised: Accepted: Published: 7237

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Table 1. Major Element Concentrations and Some Physicochemical Parameters of the Biosolid Sample24 major elements (g/kg) parameters

Ca

Fe

S

K

50.4 organic C (g/kg)

36.5 total N (g/kg)

5.9 water content (%)

90.4

16.3

66.6

1

Mg

Mn

P

2 mineral matter (dry %)

0.2 pH

11.2

37.2

10.8

diffractograms and references data from International Centre for Diffraction Data (ICDD). KNO3, HNO3, and KOH were obtained in the purest commercially available grade (Prolabo for KNO3 and Fluka for HNO3 and KOH) and used without further purification. Copper nitrate (Cu(NO3)2.5H2O, purity >98%) and nickel nitrate (Ni(NO3)2.6H2O, purity >98.5%) were purchased from Fluka, zinc nitrate (Zn(NO3)2.4H2O, purity 98%) was purchased from Merck. 2.2. Metal Adsorption Experiments. All the experiments were carried out at a fixed ionic strength equal to 0.1 M (KNO3) using batch technique at room temperature (20 °C) for the single and ternary mixed metal solutions. Adsorption edge studies were conducted to study the effect of pH in the 2− 12 pH range for each metal (Ni, Cu, and Zn). The soil (2 g) or the biosolid-amended soil sample (2 g of soil amended at 150 t/ha, preparing by mixing the soil with 5.8% of dried biosolid)24 was immersed in 15 mL of background electrolyte solution and stirred during 24 h. After this pre-equilibration step, the metallic solution was added to obtain a final concentration of 2.10−4 M in a final volume of 25 mL and the pH was incrementally adjusted to a fixed value by dropwise addition of 0.1 M HNO3 or KOH. The flasks were then shaken with an automatic shaker at room temperature for the time necessary to reach adsorption equilibrium (4 h). After filtration through a 0.45 μm cellulose acetate membrane, the supernatants were acidified with HNO3 and the metal concentration was measured by ICP-OES spectrometer (ThermoFisher ICAP 6300 DUO). The amount of adsorbed metal was deduced by difference from the initial concentration. The pH was monitored using a pH meter (Metrohm, E-654), with a combined pH electrode (Metrohm). The calibration was performed using two standard solutions of pH 4.01 and pH 7.00 at 25 °C. Competition studies were conducted in a similar way with simultaneous equimolar additions of Cu, Ni, and Zn over the same pH range for the biosolid-amended soil sample. Precipitation curves of the metallic cations as a function of pH were obtained, without solid, in the same experimental conditions. 2.3. μ-Focus XAS and XRF Measurements. Measurements were performed on the biosolid-amended soil sample simultaneously spiked by the three metallic cations (Cu, Ni, Zn). Their concentrations were the following: Cu (810 mg/kg dry weight equivalent), Ni (1250 mg/kg), and Zn (1810 mg/ kg). The biosolid-amended soil sample was air-dried, ground, and prepared as thin sections of 20 μm thickness by Thin Section Lab (Toul, France) by embedding in a resin (Huntsman, Araldite 2020 A/B). μ-XRF and μ-XANES have been achieved using a microfocus beamline (I18) at the Diamond Light Source, UK,30 with a spot size of 4 μm. All experiments were performed at room temperature with a Si(111) monochromator. Samples were recorded at Ni, Cu, and/or Zn K-edge (8333, 8979, and 9659 eV, respectively). μXRF analysis was performed using an incident beam energy of 13.6 keV and collecting the fluorescence using a 9-element Ge

focusing on biosolids only underestimated the influence of soil reactivity, which play a major role in metal behavior from biosolid-amended soils.22,23 The objective of the present work is to understand the behavior of three commonly encountered potentially toxic elements (copper, nickel, and zinc) in a biosolid-amended calcareous soil sample from the Champagne-Ardenne area (France). Batch experiments were first conducted to determine the pH-adsorption edge for the three target elements in single and multicomponent systems. Microfocus X-ray fluorescence spectroscopy (μ-XRF) was then used to produce element maps of Cu, Ni, Zn, and Fe to examine the localization of these elements in the sample, while microfocus X-ray absorption near edge structure (μ-XANES) was used to investigate the solid phase speciation of Cu, Ni, and Zn in the biosolid-amended soil. Thus, macroscopic results were combined with a molecular scale study using μ-XAS and μ-XRF techniques to characterize the availability of Cu, Ni, and Zn in the biosolid-amended soil. To our knowledge this is the first study using this approach onto a biosolid-amended soil; the relatively few studies using a similar approach deal with biosolids only.14,19,20

2. MATERIALS AND METHODS 2.1. Solids and Reagents. A natural agricultural soil from the Champagne-Ardenne area (Reims, France) was collected from 0 to 15 cm depth. Five subsamples were collected and pooled together. The soil was air-dried and sieved through 2 mm prior to perform laboratory experiments. The biosolid was provided by a large municipal wastewater treatment plant for a population equivalent of 200 000 inhabitants (France) where it was mixed with 8% of lime. The soil and biosolid (Table 1)24 physicochemical characteristics were determined by an agronomy analysis laboratory ̂ d’Analyse Marne Ardennes, Reims, France) following (Chaine French (NF) and international (ISO) standard methods. For particle size analysis, classical methods using a sedimentation pipet were applied without decarbonation (NF X31-107; AFNOR 2003).25 The methods used for the determination of pH (water), organic carbon (OC) content, cationic exchange capacity (CEC) and calcite content were ISO 10390 (2005),26 ISO 14235 (1998),27 NF X31-130 (AFNOR 1999)28 and ISO 10693 (1995),29 respectively. Organic matter (OM) content was calculated as follows: OM% = 1.72 × OC%. Powder X-ray diffraction (XRD) was performed on an ADVANCE Bruker D8 diffractometer using the Co Kα radiation (1.78 Å) for the soil and the biosolid-amended soil. The data collection was carried out from 2Θ = 3−64° with a step of 0.035° and an acquisition time of 3 s. To study the clay mineralogy, the soil sample was first decalcified with 0.2 M HCl. Then, the clay-size fraction ( Cu2+ > Zn2+. Figure 2 presents the comparison of adsorption curves as a function of pH obtained for the biosolid-amended soil and the soil sample. The pH adsorption edges of Cu were very similar for the soil and the biosolid-amended soil between pH 2 and 7. In basic medium, Cu adsorption rates decreased in both samples, but in a less extent in the soil, with the lowest adsorption capacity for the biosolid-amended soil (Figure 2). This result suggests that copper adsorption is influenced by organic matter from both the soil and the biosolid. As for Cu,

3. RESULTS AND DISCUSSION 3.1. Physicochemical Characteristics of the Soil and the Biosolid. The loamy soil was moderately basic with a pH value equal to 8.2 and is chiefly constituted by CaCO3 (81.3% on dry weight basis). The cation exchange capacity is equal to 8.2 mequiv for 100 g of soil (d.w. equivalent). The main crystalline phases resulting from XRD measurements on the soil are summarized in Table 2. XRD results confirmed the predominance of calcite and showed the occurrence of quartz, albite and aluminosilicates (chlorite, Illite, kaolinite). Among these mineral compounds, aluminosilicates are the most reactive phases in the calcareous soil. Adsorption of ions on Table 2. Mineral Phases Detected by XRD on the Calcareous Soil and the Lime-Stabilized Biosolid samples soil

biosolid

mineral phases calcite (CaCO3), quartz (SiO2), feldspar (albite: NaAlSi3O8), aluminosilicates: chlorite (clinochlore: (Mg,Fe)5Al(Si3Al) O10(OH)8), illite (KAl2(AlSi3O10)(OH)2), kaolinite (Si2Al2O5(OH)4) calcite (CaCO3), quartz (SiO2), anhydrite (CaSO4), ettringite (Ca6Al2(SO4)3(OH)12,26H2O) trace compounds: hydrotalcite (Mg6Al2(CO3)(OH)16,4H2O), Ca,Mn, and Zn phosphates, aluminum sulfate hydrate (khademite: Al(SO4)F,5(H2O)) 7239

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Figure 1. pH adsorption edges of Cu, Ni, and Zn onto the biosolid-amended soil for single and ternary metal solutions at room temperature in KNO3 0.1 M medium, with [M2+] = 2.10−4 M. Solid lines are the metal precipitation curves.

Figure 2. Comparison of Cu, Ni, and Zn pH adsorption edges onto the soil and the biosolid-amended soil as a function of pH at room temperature in KNO3 0.1 M medium, with [M2+] = 2.10−4 M.

decreased with pH in the biosolid-amended soil, it remained stable (100% adsorbed Ni) in the soil. This means that the fate of Ni was chiefly influenced by organic matter from the biosolid

the pH adsorption edges of Ni were not significantly different between pH 2 and 8 for the soil and the biosolid-amended soil (Figure 2). At pH values above 8, while Ni adsorption rates 7240

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Figure 3. Distribution of the selected elements (Ca, Fe, Ni, Cu, Zn) in the thin section of the biosolid-amended soil sample determined by μ-XRF mapping at the microscopic scale. Pixel size is 4 μm. The total map area is 750 × 750 μm2.

3.3. μ-Focus XAS and XRF Measurements. μ-XRF maps were used to find correlations among elements in the biosolidamended soil sample, pinpointing areas of interest after which μ-XANES data were collected and compared with the spectra obtained for selected standards to investigate the spatial distribution and speciation of heavy metals. Figure 3 showed the distribution of Ca, Fe, Ni, Cu, and Zn elements. Nickel, copper, and zinc were accumulated in welldefined hot spots and their distribution was rather well correlated with that of iron and in a less extent with that of calcium. Thus, these μ-XRF maps of the biosolid-amended soil demonstrated the similar distribution of Ni, Cu, and Zn at the microscopic scale. In particular, one hot spot was found exactly at the same position for each element. The overlapping of Ni, Cu, and Zn with Fe suggested a close correlation of the adsorbed metallic cations to amorphous Fe oxy(hydr)oxides in the biosolid-amended soil. The involvement of calcium phases (calcite) cannot be ruled out even if it is unlikely. Moreover, it has to be noticed that the major drawback of this technique is that carbon from organic matter cannot be observed due its low energy and to sample preparation (embedded in epoxy resin). To overcome this drawback and to obtain the metal speciation,

in the biosolid amended soil. On the contrary of Cu and Ni, the pH adsorption edges of Zn were very similar between pH 5 and 12 for the soil and the biosolid-amended soil. In acidic medium, Zn adsorption rates are higher in the biosolid-amended soil than in the soil alone, meaning that the biosolid can influence Zn adsorption capacity in the biosolid-amended soil through iron-bearing phases. Finally, the simultaneous addition of the three metallic cations did not affect the pH edge of each metallic cation (Figure 1) indicating that there is no significant competitive effect between Cu, Ni, and Zn for the biosolid-amended soil surface. To summarize, similar trends with slight differences in acidic or basic media between the soil and the biosolid-amended soil pH adsorption edge curves indicate that adsorption processes can occur onto surface sites from both the soil and biosolid components. Moreover, the biosolid components (organic matter and iron minerals) influenced metal adsorption within the biosolid-amended soil, but the mechanisms underlying this effect require an in-depth analysis using synchrotron-based techniques. 7241

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μ-XANES spectra were then recorded at Ni, Cu, and Zn “hot spots” and linear combination fittings (LCF) were performed using reference compounds (Figure 4). Three “hot-spots” were selected for each metal and the corresponding μ-XANES spectra were very similar. A step by step comparison of the different possible combinations of standard spectra was used to obtain the best fit of the sample spectra. The goodness of fit is

indicated by the R-factor. Since our biosolid-amended soil sample was heterogeneous and that the LCF results are dependent on the choice of selected standards, the results should be taken with caution and need to be combined with μXRF mapping and macroscopic data. As shown in Figure 4, the experimental μ-XANES data were well fitted by the linear combination fitting of the reference compounds (Table 3). In the case of Ni, the LCF results Table 3. Linear Combination Fits of Ni, Cu, and Zn μXANES Spectra from the Biosolid-Amended soil (Fig. 4) spectrum Ni Cu Zn

metal hydroxide (%)

metal− OM (%)

metal−iron oxide (%)

Sum (%)

R-factor

19 59

72 41 42

9

100 100 100

0.0010 0.0006 0.0017

58

(Figure 4A) evidenced the main contribution of organic matter (72%, R-factor = 0.0010) and in a lesser extent the ones of Ni hydroxide (Ni(OH)2, 19%) and of Ni-iron oxide (9%). This result evidenced a strong degree of association with organic matter. Concerning zinc (Figure 4C), the LCF of the XANES spectrum led to 42% of Zn bound to organic matter and 58% bound to iron oxide (R-factor = 0.0017), corresponding to an association to organo-mineral phases in the sludge-amended soil in accordance with the results obtained by Donner et al. in biosolids.19,20 Finally, for copper the association to organic matter was relatively limited (41%) and the remaining 59% were found in the Cu2(CO3)(OH)2 form (R-factor = 0.0006). This last result differs from the data obtained by Donner et al.19,20 in “pure” biosolids (i.e., without soil). Nevertheless, this is not surprising since our biosolid-amended soil is calcareous with a high pH value (around 8). To summarize, LCF data indicated consistent differences between the metal geochemical partitioning leading to strong differences in term of mobility in biosolid-amended soils. This result contrasts with those obtained by μ-XRF due to the inability to identify the organic material by this technique. 3.4. Implications for Long-Term Metal Mobility in Biosolid-Amended soils. Together, pH-adsorption edge experiments combined with synchrotron-based spectroscopy techniques indicate consistent differences in metal speciation between metals. While organic matter plays a dominant role in Ni binding in the biosolid-amended soil, it was of lesser importance for Cu and Zn which were predominantly present as carbonato-hydroxide or bound to organo-mineral phases (iron-bearing phases), respectively. Moreover, this study suggests that even if the metals can be associated with soil components (clay minerals and organic matter), biosolid application will increase metal retention in the biosolidamended soil by providing reactive organic matter and iron oxide fractions. Among the studied metals, the long-term mobility of Ni could be affected by organic matter degradation (“time bomb” hypothesis), while Cu and Zn are strongly stabilized, and thus less mobile (“protection” hypothesis) in biosolid-amended soils through the formation of more or less insoluble phases or association with iron oxides. This study reinforces the conclusions drawn from biosolids alone14,19,20 about the mobility of metals in soils amended with biosolids and clearly demonstrates the benefits of using a multiscale approach.

Figure 4. μ-XANES spectra and XANES linear combination fits (LCF) using standard compounds listed in the experimental section of Ni (A), Cu (B), and Zn (C) hotspots in the biosolid-amended soil. All weights are between 0 and 1 and their sum is equal to 1. 7242

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AUTHOR INFORMATION

Corresponding Author

*Phone: +33(0)3 26 91 32 43; fax: +33 (0)3 26 91 32 43; email: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the EU ROUTES project (Contract No 265156, FP7 2007-2013, THEME [ENV.2010.3.1.1-2] Innovative system solutions for municipal sludge treatment and management). This work was made possible through a beam time award from Diamond Light Source Ltd. (ref No. SP8012). A. Goupille is acknowledged for ICP-OES measurements.



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