Integrated Approaches of X‐Ray Absorption Spectroscopic and Electron Microscopic Techniques on Zinc Speciation and Characterization in a Final Sewage Sludge Product

Abstract

Integration of complementary techniques can be powerful for the investigation of metal speciation and characterization in complex and heterogeneous environmental samples, such as sewage sludge products. In the present study, we combined analytical transmission electron microscopy (TEM)-based techniques with X-ray absorption spectroscopy (XAS) to identify and characterize nanocrystalline zinc sulfide (ZnS), considered to be the dominant Zn-containing phase in the final stage of sewage sludge material of a full-scale municipal wastewater treatment plant. We also developed sample preparation procedures to preserve the organic and sulfur-rich nature of sewage sludge matrices for microscopic and spectroscopic analyses. Analytical TEM results indicate individual ZnS nanocrystals to be in the size range of 2.5 to 7.5 nm in diameter, forming aggregates of a few hundred nanometers. Observed lattice spacings match sphalerite. The ratio of S to Zn for the ZnS nanocrystals is estimated to be 1.4, suggesting that S is present in excess. The XAS results on the Zn speciation in the bulk sludge material also support the TEM observation that approximately 80% of the total Zn has the local structure of a 3-nm ZnS nanoparticle reference material. Because sewage sludge is frequently used as a soil amendment on agricultural lands, future studies that investigate the oxidative dissolution rate of ZnS nanoparticles as a function of size and aggregation state and the change of Zn speciation during post sludge-processing and soil residency are warranted to help determine the bioavailability of sludge-born Zn in the soil environment.

Highly-defective nanocrystals of ZnS formed via dissimilatory bacterial sulfate reduction: A comparative study with their abiogenic analogues

“…Counter-intuitively, evidence shows that nano-metal sulfides are not confined to anoxic environments as colloids of ZnS, CdS, and CuS can persist in oxic waters for months, transporting metals between different environments and ecological niches via metal-sulfide associations (Theberge et al, 1997;Rozan et al, 2003;Sukola et al, 2005;Priadi et al, 2012). In addition, recent environmental studies confirmed the presence of ZnS nanoparticles in domestic and industrial sewage and sludge (Lombi et al, 2012;Kim et al, 2014;Ma et al, 2014), and suggested that sulfidation is a dominant process for the dissolved metals in these engineered systems.…”

Anaerobic Digestion Alters Copper and Zinc Speciation

“…Indeed, Zn sulfide minerals have only been observed in OW in the few studies using XAS. 6,29 Standard sequential chemical extraction methods underestimate the sulfide fraction, since it is extracted along with OM fractions. 34 Zn species bound to organic species were only present in a minor proportion (12%) in MWFF, thus confirming the minor role played by OM in Zn speciation in OW.…”

Outdoor urban nanomaterials: The emergence of a new, integrated, and critical field of study

“…Also numerous studies demonstrated the sulfidation of different types of ENMs (e.g. Ag and ZnO) in WWTPs, limiting the release of ionic species (Kim et al, 2014;Kim et al, 2010;Levard et al, 2012). However, due to the obvious differences in the chemistry of stormwater and wastewater, NMs should be expected to undergo different transformations in stormwater systems compared to waste water systems and within WWTPs (Barton et al, 2015;Brar et al, 2010;Doolette et al, 2013;Kaegi et al, 2013;Kaegi et al, 2011), and the rates of these transformation processes are likely to be different, requiring further research.…”