ARTICLE pubs.acs.org/est
X-ray Absorption and Micro X-ray Fluorescence Spectroscopy Investigation of Copper and Zinc Speciation in Biosolids Erica Donner,†,‡,* Daryl L. Howard,§ Martin D. de Jonge,§ David Paterson,§ Mun Hon Cheah,|| Ravi Naidu,†,‡ and Enzo Lombi†,‡ †
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Centre for Environmental Risk Assessment and Remediation, University of South Australia, Building X, Mawson Lakes Campus, South Australia 5095, Australia ‡ CRC CARE, PO Box 486, Salisbury, South Australia 5106, Australia § Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia Australian National Beamline Facility, KEK-PF, 1-1 Oho, Tsukuba, Ibaraki, 305-0801 Japan
bS Supporting Information ABSTRACT: Despite its pivotal role in determining the risks and time frames associated with contaminant release, metal speciation remains a poorly understood aspect of biosolids chemistry. The work reported here used synchrotron-based spectroscopy techniques to investigate the speciation of copper and zinc in a range of Australian biosolids. High resolution element mapping of biosolids samples using micro X-ray fluorescence spectroscopy revealed considerable heterogeneity in key element associations, and a combination of both organic and inorganic copper and zinc binding environments. Linear combination fitting of K-edge X-ray absorption spectra indicated consistent differences in metal speciation between freshly produced and stockpiled biosolids. While sulfide minerals play a dominant role in metal binding in freshly dewatered biosolids, they are of lesser importance in dried biosolids that have been stockpiled. A degree of metal binding with iron oxide minerals was apparent but the results did not support the hypothesis that biosolids metals are chiefly associated with iron minerals. This work has potential implications for the long-term stability of metals in biosolids and their eventual fate following land application.
’ INTRODUCTION In recent years, a prevailing trend toward increasingly stringent effluent standards has prompted widespread upgrading of wastewater treatment systems. Aside from improving effluent quality, this also tends to increase the quantity of sludge produced as a byproduct of the treatment process. With large-scale disposal of sewage sludge already presenting a major management issue for the wastewater industry, it is clear that uncertainties regarding the sustainability of key disposal pathways need to be resolved. Various disposal options for sewage sludge are available but the application of stabilized sludge (i.e., biosolids) to agricultural and forestry land plays a key role, offering the advantage of recycling nutrients and organic matter from waste material back to the land. The associated benefits in terms of soil structure and nutrient status are well documented. However, biosolids also contain a range of both organic and inorganic contaminants, and the sustainability and long-term effects of land application are still being investigated.1 3 In particular, the presence of inorganic contaminants (e.g., metals) has given cause for concern due to their accumulative and persistent nature. Metal contaminants in biosolids are thought to be sorbed by both organic and inorganic constituents but debate regarding the r 2011 American Chemical Society
nature and stability of key sorbent phases is ongoing.4 While some authors have suggested that organic matter is the major sorbent5,6 others have indicated that inorganic constituents such as iron (Fe) and manganese (Mn) oxides play the dominant role.7,8 If the prevailing sorbent is indeed organic matter then, due to mineralization processes, metals may be progressively released from biosolids over time. The potential for this to occur prompted the development of a “time bomb” hypothesis5,6 by which a long-term increase in contaminant bioavailability is envisaged. Of course, it is also possible that any metals released may be redistributed to alternative binding sites, in which case the identity of these ligands is also of interest. Alternatively, if contaminants are stably sorbed by inorganic biosolids constituents their behavior should not change substantially in the long term, so long as the overall conditions (e.g., pH) in the sludgeamended soils do not change. One area of knowledge that is lacking in this discussion is a mechanistic understanding of the key metal species present in Received: December 20, 2010 Accepted: July 27, 2011 Revised: July 27, 2011 Published: July 27, 2011 7249
dx.doi.org/10.1021/es201710z | Environ. Sci. Technol. 2011, 45, 7249–7257
Environmental Science & Technology
ARTICLE
Table 1. Chemical Characteristics of Biosolids Samples (n = 1 for pH, Total C, N, and S; n = 3 for Al, Fe, P, Cu, Zn, Mean ( SE) total C
total N
total S
Al
Fe
P
Cu
Zn
sample ID
treatment
pH
(%)
(%)
(%)
(%)
(%)
(%)
(mg kg 1)
(mg kg 1)
B1
fresh biosolids (mixed primary sludge and waste
5.6
37
6.6
0.70
1.0 ( 0.03
1.3 ( 0.2
5.3 ( 0.2
286 ( 4
567 ( 6
6.6
25
3.5
0.78
3.7 ( 0.06
4.1 ( 0.06
1.8 ( 0.00
530 ( 16
5769 ( 66
6.8
29
4.4
1.09
5.1 ( 0.07
1.8 ( 0.01
3.6 ( 0.04
702 ( 14
768 ( 13
5.6
36
6.1
1.44
0.9 ( 0.02
0.9 ( 0.02
2.1 ( 0.01
871 ( 22
6.2
18
1.8
0.76
4.2 ( 0.09
3.3 ( 0.07
0.8 ( 0.03
604 ( 32
825 ( 12
6.6
12
1.5
0.69
6.8 ( 0.09
3.1 ( 0.04
1.8 ( 0.07
477 ( 17
467 ( 16
activated sludge), mechanically dewatered B2
fresh biosolids (mixed primary sludge and waste activated sludge), anaerobically digested, mechanically dewatered
B3
fresh biosolids (mixed primary and waste activated sludge), anaerobically digested, mechanically dewatered
B4
stockpiled biosolids from secondary treatment
1035 ( 5
process (dewatered mixed primary and secondary sludge) B5
stockpiled drying bed biosolids from extended aerobic and anaerobic lagoon system
B6
stockpiled (composting) biosolids from secondary treatment plant. This is an aged sample from the same treatment system as B3 (i.e., mixed primary and waste activated sludge, anaerobically digested, and centrifuged prior to composting)
biosolids. Although the potential for long-term changes in metal bioavailability can be assessed using traditional experimental methods such as long-term field trials, a mechanistic understanding of key sorption reactions and phases can provide useful information for decision making within a much shorter time frame and without the complications and costs of highly labor intensive field experiments. Synchrotron radiation techniques (e.g., X-ray Absorption Spectroscopy, XAS and X-ray Fluorescence Microscopy, XFM) are the best available techniques for in situ examination of metal speciation and associations in complex environmental media.9 By contrast, hyphenated techniques such as Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) require extraction prior to analysis, while other techniques such as Secondary Ion Mass Spectroscopy (SIMS) do not provide speciation information. Meanwhile, non synchrotron-based techniques that can be used for element mapping such as Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) lack the sensitivity to comprehensively map metals of environmentally relevant concentrations in biosolids. Yet despite the strengths of available synchrotron techniques only two published papers have reported synchrotron-based analysis of metals in biosolids. Nagoshi et al.10 examined a single sewage sludge sample using XAS, while Hettiarachchi et al.8 used micro X-ray fluorescence (μ-XRF) to examine two North American biosolids. This analysis was limited to the collection of two maps of 400 400 and 220 120 μm collected with a step size of 10 μm. The work reported here used synchrotron-based methods to provide insight into the distribution and solid-phase speciation of copper (Cu) and zinc (Zn) in six Australian biosolids. The selected sludges differed in their provenance and degree of postproduction processing and were selected to allow comparison between metal speciation in both fresh (dewatered) and aged (dried/composted) biosolids. As biosolids are currently applied either as fresh cake or dried material, it is important to determine the implications of this with regards to metal bioavailability. XAS techniques, including both X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS), were used to investigate the average speciation of Zn and Cu in the biosolids,
while μ-XRF was used to produce element maps of Zn, Cu, Fe, and Mn and to examine the colocation of these elements in the biosolids. Use of the state-of-the-art 384-element Maia XRF detection system11 drastically reduced the constraints on data collection and facilitated the generation of large (4000 4000 μm), high resolution (2 μm spot size, 2 μm step size) Megapixel maps.
’ MATERIAL AND METHODS Biosolids Samples. Biosolids samples were sourced from secondary wastewater treatment plants in five different states and territories of Australia. Biosolids were dried on receipt at 40 °C and were of two basic types. Biosolids 1, 2, and 3 (B1, B2, B3) were sampled straight from the sludge dewatering stage of secondary treatment systems and are referred to as “fresh” biosolids, whereas biosolids 4, 5, and 6 (B4, B5, B6) were obtained from drying beds and/or composting stockpiles and are referred to as “aged” biosolids. Biosolid B3 and B6 were produced by the same wastewater treatment plant. All aged samples were stockpiled for 6 to 24 months. Selected chemical characteristics of the six biosolids are presented in Table 1. Total C, N, and S were measured by combustion in a LECO CNS elemental analyzer, pH was measured in 0.01 M CaCl2 solution (1:5 solid: solution), and aluminum (Al), phosphorus (P), Fe, Cu, and Zn were measured by ICP-OES following digestion in reverse aqua regia (1:3 HCl:HNO3). Quality control measures included the use of procedural blanks and analysis of a certified sludge reference material (NIST SRM 2781). The 3-step BCR sequential extraction procedure12 was employed to fractionate Cu and Zn in acid extractable, reducible and oxidizable pools. This analysis was conducted in triplicate. Laterally Resolved XRF Elemental Maps. The sludge samples were ground using a mortar and pestle, sieved to
