Article pubs.acs.org/est
Effects of Chemical Amendments on the Lability and Speciation of Metals in Anaerobically Digested Biosolids Erica Donner,*,†,‡ Gianluca Brunetti,†,§ Bernie Zarcinas,†,‡ Paul Harris,∥ Ehsan Tavakkoli,† Ravi Naidu,†,‡ and Enzo Lombi†,‡ †
Centre for Environmental Risk Assessment and Remediation, University of South Australia, Building X, Mawson Lakes Campus, Mawson Lakes,South Australia 5095, Australia ‡ CRC CARE, PO Box 486, Salisbury, South Australia 5106, Australia § DISAAT, Department of Agricultural and Environmental Sciences, University of Bari, 165/A Via Amendola, 70126 Bari, Italy ∥ School of Agriculture, Food and Wine, University of Adelaide, PMB 1, Glen Osmond, South Australia 5064, Australia S Supporting Information *
ABSTRACT: The interaction of inorganic contaminants present in biosolids with iron, aluminum, and manganese oxy/hydroxides has been advocated as a key mechanism limiting their bioavailability. In this study, we investigated whether this is indeed the case, and further, whether it can be exploited to produce optimized biosolids products through the addition of chemical additives during sewage sludge processing. Experiments were conducted to investigate whether the addition of iron- and aluminum-based amendments (at 5 different rates) during the anaerobic digestion phase of wastewater treatment can effectively change the speciation or lability of contaminant metals (copper, zinc and cadmium) in biosolids destined for use in agriculture. The performance of the bioreactors was monitored throughout and the speciation and lability were determined in both fresh and 3-month aged biosolids using X-ray absorption spectroscopy (Cu, Zn) and isotopic dilution (65Cu, 65 Zn, 109Cd). The tested amendments (FeCl3, Al2(SO4)3, and Al-rich water treatment residual) did not cause significant changes in metal speciation and were of limited use for reducing the lability of contaminant metals in good quality biosolids (suitable for use in agriculture), suggesting that high affinity binding sites were already in excess in these materials. However, the use of chemical amendments may offer advantages in terms of treatment process optimization and may also be beneficial when biosolids are used for contaminated site remediation.
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Oliver et al.7 used isotopic exchangeability (E-values) to investigate copper (Cu) lability in biosolids from 18 different wastewater treatment plants and reported the proportion of labile Cu to be less than 43% of the total Cu in all samples. Donner et al.3 reported 37−74% Cu lability, and 31−48% zinc (Zn) lability for samples encompassing both high quality contemporary biosolids (438 and 429 mg kg−1 Cu and Zn) and highly contaminated historic biosolids (up to 1121 and 2945 mg kg−1 Cu and Zn). Some studies have also indicated that metals added to soils as components of biosolids may be less soluble and/or phytoavailable than metals added as soluble metal salts;8,9,5 however, other studies have generated conflicting results.10,11 The issue is certainly complex because of the wide variety of different metals, biosolids, soil types, and time scales, which need to be considered.
INTRODUCTION The use of biosolids (stabilized sewage sludge) on agricultural soils offers an opportunity to recycle nutrients that would otherwise be lost from the food production cycle and is particularly beneficial for soils with low organic carbon content. To mitigate the risks associated with the land application of biosolids this practice is statutorily regulated in many countries, including the U.S.A., EU member states, and Australia, where approximately 36%, 45%, and 60% of biosolids are used in agriculture, respectively.1 Aside from reducing pathogenic risks, the major purpose of these regulations is to protect soil quality by limiting the addition of contaminants (e.g., metals) to soils. Although the advent of more stringent pollution permitting schemes and source control programs has effectively lowered the concentrations of metals in biosolids over the past few decades,2,3 metal contaminants are still of regulatory concern and the suitability of existing limits remains a subject of research and debate.4−6 Previous research has shown that the metals contained in biosolids are only partially chemically labile and bioavailable. © 2013 American Chemical Society
Received: Revised: Accepted: Published: 11157
February 21, 2013 July 29, 2013 August 27, 2013 August 27, 2013 dx.doi.org/10.1021/es400805j | Environ. Sci. Technol. 2013, 47, 11157−11165
Environmental Science & Technology
Article
Table 1. Chemical Amendment Treatments Tested in This Experiment and Mean pH, Eh, and Temperature Values Measureda amendment tested ferric chloride
aluminum sulfate
water treatment residual (WTR)
treatment ID control treatment treatment treatment treatment treatment control treatment treatment treatment treatment treatment control treatment treatment treatment treatment treatment
1 2 3 4 5
Fe added (mmol L−1)
mean pHb
mean Ehb
mean temp (°C)b
mean biogas produced (L gas L−1 sludge digested)
0.0 2.2 4.5 9.0 17.9 35.8 0.0 2.2 4.5 9.0 17.9 35.8
6.9 6.9 6.9 6.9 6.8 6.5 6.9 6.8 6.6 6.7 6.4 6.3 6.9 6.9 6.9 6.9 6.8 6.9
−490 −491 −492 −493 −476 −465 −492 −500 −500 −494 −463 −468 −452 −453 −450 −449 −593 −464
36.1 35.6 34.8 35.7 35.5 35.6 36.2 36.3 35.3 35.8 35.5 36.3 38.3 38.0 37.1 37.9 37.7 37.7
7.9 9.7 10.3 10.0 10.3 8.0 8.4 8.0 6.6 4.7 3.5 3.0 5.7 6.5 5.7 6.2 6.4 5.8
0 2.2 4.5 9.0 17.9 35.8
1 2 3 4 5 1 2 3 4 5
Al added (mmol L−1)
0 0.03 0.06 0.12 0.24 0.48
a
Recorded by datalogger every 15 minutes throughout the anaerobic digestion experiment; n = 2083. bStandard error of the mean calculated across all treatments (n = 2083) was ≤0.01 for pH measurements; ≤ 1.3 for Eh measurements, ≤ 0.09 for temperature measurements; and ≤1.5 for biogas quantity produced.
coagulating agents (e.g., Al salts) and has been shown to be an effective cation and anion fixation agent.19 Iron-rich industrial waste streams are also available and could potentially be substituted for FeCl3.20,21 Thus ideally, the modified treatment process would not only reduce the environmental risks associated with the agricultural use of biosolids, it could also provide an alternative avenue for the sustainable disposal of other waste products. The use of chemical amendments may also offer additional advantages in terms of treatment process optimization. For example, FeCl3 addition has been shown to decrease the concentration of dissolved H2S in digesters, thereby increasing the efficiency of methanogenesis and improving the quality of the biogas produced.22,23 A combination of isotopic dilution methods and X-ray absorption spectroscopy (XAS) was used to investigate whether the tested chemical amendments produced measurable changes in Cu, Zn, and Cd lability in biosolids, and whether any measurable changes in lability could also be linked to changes in chemical speciation. Furthermore, as metal speciation changes during postproduction processing of biosolids via stockpiling and composting,16 both fresh and aged biosolids were produced and analyzed for all treatments considered in this experiment. Key anaerobic digestion indicators, such as biogas quality and quantity, were also monitored during this study.
As the physicochemical properties of biosolids are expected to influence the speciation and retention/release of biosolidsborne metals,12 one obvious direction of research is to investigate whether biosolids production and treatment processes can be adapted to produce biosolids optimized for agricultural application. For instance, if the contaminant metals could be tightly bound by stable mineral phases in the biosolids matrix they may effectively be biologically inert. This could introduce an extra safety factor to protect soil and crop quality, and may also limit changes in long-term metal bioavailability following land application. Several researchers have suggested that metal bioavailability in biosolids or biosolids amended soils may be reduced due to interactions with key mineral phases, such as Fe-, Mn-, and Al-oxides.9,13−15 Recent research has also shown that biosolids metals such as Cu and Zn have differing speciation in fresh and aged biosolids, with redistribution occurring during the stockpiling/composting phase.16 These results suggest that by changing the relative proportion of mineral forming elements present during key mineral formation stages (e.g., anaerobic digestion or composting) it may be possible to alter the chemistry of the final biosolids product and reduce the lability of metal contaminants in the long term. However, the potential to manipulate metal availability by changing the concentrations of key mineral components during biosolids processing has not yet been systematically investigated. The main objective of this study was to investigate whether routine dosing of Fe and Al during biosolids production could influence the speciation of metals in the final product and effectively reduce contaminant metal lability. The experiment was focused on the manipulation of the anaerobic digestion process as that is one of the most widely favored and effective methods of sewage sludge stabilization.17,18 Three potential chemical amendments were investigated: ferric chloride (FeCl3), aluminum sulfate (Al2(SO4)3, and aluminum-rich water treatment residual (WTR). WTR is a large-volume waste product produced during the treatment of drinking water with
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MATERIALS AND METHODS Chemical Amendments. The chemical amendments examined in this study were FeCl3, Al2(SO4)3, and Al-rich WTR. Analytical grade FeCl3 and Al2(SO4)3 were obtained from BDH and Sigma Aldrich, respectively, and liquid WTR was sampled from the influent pipe to a WTR drying bed at the Barossa Water Treatment Plant (South Australia) two days prior to commencing the experiment. The Al and Fe concentrations in the WTR were immediately determined by reverse aqua regia (3:1 HNO3/HCl) digestion and ICP OES analysis (the WTR contained 56 mg kg−1 Cu and 6.6 mg kg−1 11158
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Environmental Science & Technology
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shaker. The suspensions were then triple-spiked with 25 KBq Cd, 50 KBq 65Zn, and 22 μg 65Cu and shaken for a further 72 h before centrifugation and filtration (0.22 μm). Metal concentrations in solution were measured using ICP-OES and ICP-MS. A gamma counter was used for radio-assay. Stable isotope ratios were measured as documented by Zarcinas et al.26 Zinc and Cd E-values were calculated as described in Hamon et al.25 and Cu E-values as described by Nolan et al.27 The pH of the equilibrated samples was also measured. The SPSS statistical software package was used to perform ANOVA and least significant difference (LSD) analyses on the E-value data set. X-ray Absorption Spectroscopy (XAS). XAS is a powerful technique capable of examining metal speciation in situ in solid biosolids samples,28 and was used to investigate whether the chemical amendments and/or biosolids aging process induced changes in Cu and Zn speciation (Cd concentrations were too low for XAS analysis). The samples selected for this analysis included a control plus the second and fifth treatment levels from the FeCl3 and Al2(SO4)3 amendment trials, and both fresh and aged samples were analyzed. Duplicate biosolids samples from each treatment were finely ground and combined for analysis. Biosolids Cu and Zn K-edge X-ray absorption near edge structure (XANES) spectra were collected at room temperature in fluorescence mode on the XAS beamline at the Australian Synchrotron. The electron storage ring operated at 3 GeV in top-up mode. Elemental Cu and Zn foils were used for energy calibration (Cu K-edge at 8979 eV and Zn K-edge at 9659 eV) by collecting transmission spectra for the relevant foil at the same time as the sample fluorescence spectra (3 replicate scans per sample). The beamline setup used a liquid nitrogen cooled Si (111) crystal monochromator and the fluorescence data were collected using a 100 element Ge fluorescence detector. Standard spectra used for Linear Combination Fitting (LCF) are shown in Supporting Information Figures S1 and S2 and include: chalcopyrite, covelite, Cu-phosphate, Cu sorbed on humic acid, Cu sorbed on gibbsite, Cu-sorbed ferrihydrite, Zn-cysteine, Zn-sorbed ferrihydrite, hopeite, sphalerite, wurtzite, Zn sorbed on humic acid, Zn sorbed on gibbsite, and a ZnAl layered double hydroxide silicate mineral. All samples were analyzed as finely ground powders. Standard materials were diluted in boron nitride to approximately 1000 mg kg−1. The principal component analysis (PCA) and target transformation (TT) approach for XAS data analysis was used to select spectra for LCF fitting of the Zn and Cu XANES spectra.29 PCA and TT were performed using SixPack while data normalization and LCF were performed using Athena. The fitting range was −100 to +150 eV relative to the Cu or Zn K-edge. For each sample, the combination of standards with the lowest LCF residual parameter was chosen as the most likely set of components.30
Zn). On the basis of this analysis, WTR was added to the bioreactors at rates calculated to provide equivalent Al masses to those used in the Al2(SO4)3 amendment trial. Anaerobic Digestion, Monitoring, and Chemical Analysis. A mixture of primary and secondary sludge from a large-scale wastewater treatment plant in South Australia was anaerobically digested in benchtop bioreactors in a 35 °C constant temperature room. The experiment was carried out in three phases, with one 22 day batch digestion phase for each different chemical amendment experiment (FeCl3, Al2(SO4)3, WTR). Each test involved five different treatment levels of the target amendment and one control treatment (Table 1). Equivalent molar ratio masses of Fe and Al were added to the digesting sludge to facilitate direct comparison between amendments. Duplicate bioreactors were run for each treatment, giving a total of 36 bioreactors. Each 2 L bioreactor was fed with a 60:40 mixture of primary and secondary sewage sludge and seeded with 50 mL of anaerobically digested sludge to help establish the bacterially driven anaerobic digestion process. One of the two duplicate bioreactors for each treatment level was equipped with pH, redox and temperature probes; these parameters were recorded via datalogger every 15 min throughout the experiment. Gas was collected in 1 L Tedlar gas sampling bags for volume and quality monitoring. The bioreactors were continuously stirred using a magnetic stirrer to ensure thorough mixing of the substrate during digestion. Sodium bicarbonate was added to all bioreactors to establish a suitable alkalinity level prior to start-up of the digesters in order to maintain process stability24 and counteract the acidification potential of the Al and Fe salts. Variables monitored during the anaerobic digestion phase included pH, Eh (redox potential), temperature, biogas volume produced, and biogas composition and quality (e.g., methane content). Following digestion the samples were centrifuged to separate the solid and liquid fractions. Solids were then dried at 40 °C and finely ground. Subsamples of these materials (hereafter referred to as “fresh” biosolids) were kept in sealed plastic vials, while the remainder were subjected to a 3-month regime of wetting, drying, and aging at 37 ± 2 °C. The resulting samples, hereafter, referred to as “aged” biosolids, were also dried at 40 °C and finely ground. Biosolids Analysis. Subsamples of all biosolids were digested in reverse aqua regia (3:1 HNO3:HCl) and analyzed by ICP-OES and ICP-MS for Cu, Zn, Cd, Fe, and Al. Quality control measures included the use of procedural blanks and analysis of a certified reference material (NIST SRM 2781, domestic sludge). The lability (isotopic exchangeability) of Cu, Zn, and Cd in each fresh and aged sample was investigated using isotopic dilution techniques and the Zn and Cu speciation of selected samples was investigated by X-ray absorption near edge spectroscopy (XANES). These methods are described in detail below. Isotopic Dilution with 65Zn, 109Cd, and 65Cu (E-Values). Isotopic dilution was used to investigate the proportion of labile (potentially bioavailable) Cd, Zn, and Cu in all biosolids samples. The isotopically exchangeable fraction thus determined is termed an E-value; a detailed theoretical discussion of this technique can be found in Hamon et al.25 Cadmium and Zn E-values were calculated on the basis of radioisotope dilution with 109Cd and 65Zn, while Cu E-values were calculated using stable isotope dilution with 65Cu. All measurements were carried out in triplicate. Each 1.2 g of biosolid sample was equilibrated in 12 mL of 0.01 M CaCl2 for 24 h on a rotating
109
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RESULTS AND DISCUSSION Bioreactor Monitoring Parameters and Biosolids Metal Contents. A summary of all chemical amendment treatments tested in these experiments is presented in Table 1, together with the results of key digester monitoring parameters (i.e., pH, Eh, temperature, and biogas volume). Measured concentrations of Fe and Al in the final biosolids products ranged from 1.6 to 10.4% Fe and from 4.0 to 8.2% Al. This represents an increase in Fe and Al concentrations in the highest amendment levels of 4.2% Al relative to the control treatment and 8.8% Fe relative to the control treatment, which 11159
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confirms that the Fe and Al amendments were added on an equivalent molar basis. Mean Cu, Zn, and Cd concentrations (n = 108) in the fresh biosolids were 501 (SE = 5.4), 619 (SE = 7.4), and 1.02 mg kg−1 (SE = 0.01) respectively. Redox potential (Eh) was logged every 15 min throughout the experiment and showed that highly anaerobic conditions were maintained in all treatments throughout the digestion process. A slight trend of decreasing pH (max 0.6 pH units) with increasing amendment level was recorded in both the FeCl3 and Al2(SO4)3 amendment trials, but not in the WTR amendment trial. The biogas volumes produced suggested that midrange FeCl3 treatments may have had a positive effect on the bioreactor efficiency, but detrimental effects were noted in the case of Al2(SO4)3 amendments. This may have been partly due to the pH gradient observed in these bioreactors as this trend of decreasing biogas with increasing Al addition was not evident in the WTR trial and the WTR treatments did not significantly affect pH. Optimum pH for the anaerobic digestion process differs depending on the solids content and sludge characteristics but is typically reported to be around 6.8−7.4.31 Isotopic Exchangeability of Cu, Zn, and Cd. Isotopic dilution was used to assess whether the addition of chemical amendments during sludge digestion reduced the lability of contaminant metals in the biosolids. Fresh and aged biosolids samples were analyzed in order to test for potential effects both during and after the anerobic digestion process. Changes were considered likely to occur during the aging process as the sulfide minerals present in the fresh biosolids16,32 should progressively oxidize during this period, and as the sample mineralogy changes, contaminant metals may coprecipitate with newly forming metal (hydr)oxide phases (i.e., Al and Fe minerals). The results of the isotopic exchangeability analyses are presented in Figures 1−3, where the E-values (i.e., isotopically
Figure 2. Average zinc lability (expressed as a percentage of the total zinc content) in the fresh and aged biosolids produced from the 18 bioreactor treatments (n = 6; 3 isotopic dilution measurements per bioreactor; two replicate bioreactors per treatment).
Figure 3. Average cadmium lability (expressed as a percentage of the total cadmium content) in the fresh and aged biosolids produced from the 18 bioreactor treatments (n = 6; 3 isotopic dilution measurements per bioreactor; two replicate bioreactors per treatment).
amendments. In fact, the only significant and consistent difference between treatments (i.e., a change that consistently follows the increasing addition of amendments) was in the case of the Cu E-values in the fresh sludge, where the lability of the Al2(SO4)3 treatment was lowest (P < 0.001). Nevertheless, this difference disappeared in the aged biosolids. In contrast, the lability of each of the three contaminant metals increased significantly (P < 0.001) upon aging of the biosolids across all treatments. This increase in lability is linked to changes in metal speciation which have been reported to occur as a result of the composting/aging process, as metals locked away in precipitated sulfide minerals are released because of oxidation processes during aging and redistribute onto alternative binding sites within the biosolids matrix.16 The results from this experiment show that the isotopic exchangeability of the metals significantly increases as a result of the redistribution process. Figure 1 shows that Cu lability ranged from ∼10 to 50% for all amendments and treatments, including the controls, and clearly increased as a result of the aging process. There was no consistent trend in Cu lability for fresh biosolids from the FeCl3 and WTR treatments. There was however a statistically significant decrease in Cu lability in the fresh biosolids obtained with the addition of Al2(SO4)3. In this case, the Cu lability in
Figure 1. Average copper lability (expressed as a percentage of the total copper content) in the fresh and aged biosolids produced from the 18 bioreactor treatments (n = 6; 3 isotopic dilution measurements per bioreactor; two replicate bioreactors per treatment).
exchangeable fraction) are expressed as the percentage of total labile metal (Cu, Zn, or Cd, respectively). In each figure, the average lability of the target metal in biosolids from all control and amendment treatments is presented for both fresh and aged biosolids samples. Collectively, Figures 1−3 clearly show that, across all treatments, contaminant metal lability was affected more by the aging process than by the added 11160
dx.doi.org/10.1021/es400805j | Environ. Sci. Technol. 2013, 47, 11157−11165
Environmental Science & Technology
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Figure 4. Normalized Cu and Zn K-edge XANES spectra for selected samples analyzed from the fresh and aged ferric chloride and aluminum sulfate biosolids treatments (solid lines). Dotted lines show the best linear combination fit of reference spectra as documented in Table 2
the control and lower dose of Al2(SO4)3 addition was lower than in the two intermediate treatments; and the two highest treatments showed the lowest Cu lability (P < 0.001). In the aged samples, all amendments showed a slight decrease in Cu lability at the highest treatment levels, but in all cases the difference was not statistically significant and
