Non-labile silver species in biosolids remain stable throughout 50 years of weathering and ageing

Abstract

Increasing commercial use of nanosilver has focussed attention on the fate of silver (Ag) in the wastewater release pathway. This paper reports the speciation and lability of Ag in archived, stockpiled, and contemporary biosolids from the UK, USA and Australia, and indicates that biosolids Ag concentrations have decreased significantly over recent decades. XANES revealed the importance of reduced-sulfur binding environments for Ag speciation in materials ranging from freshly produced sludge to biosolids weathered under ambient environmental conditions for more than 50 years. Isotopic dilution with (110 m)Ag showed that Ag was predominantly non-labile in both fresh and aged biosolids (13.7% mean lability), with E-values ranging from 0.3 to 60 mg/kg and 5 mM CaNO3 extractable Ag from 1.2 to 609 μg/kg (0.002-3.4% of the total Ag). This study indicates that at the time of soil application, biosolids Ag will be predominantly Ag-sulfides and characterised by low isotopic lability.

Nanomaterials in the environment: Behavior, fate, bioavailability, and effects—An updated review

“…As with anoxic aquatic systems, a wide variety of Ag NMs will largely transform into insoluble Ag 2 S NMs in a range of WWTP configurations (Lombi et al ; Ma et al ; Pradas del Real et al ; Wang et al ), although Ag‐cysteine and AgCl are also possible transformation end products. Considering that non‐labile transformation end products such as AgCl and Ag 2 S are expected to be stable in the environment and to have relatively low bioavailability (Lombi et al ; Donner et al ; Doolette et al , ), the risk to terrestrial ecosystems posed by Ag NMs would appear to be low, although uncertainties related to longer‐term transformations, dosimetry, are also relevant. This conclusion has been further supported by soil‐based studies demonstrating that Ag 2 S NMs presented minimal hazard to plant–microorganism symbioses (Judy et al , ), crop plants (Doolette et al ; Wang et al ), soil microorganisms (Judy et al ; Doolette et al ; Moore et al ), and soil invertebrates (Starnes et al , ).…”

Fate of metallic engineered nanomaterials in constructed wetlands: prospection and future research perspectives

“…(2012a) noticed however that sulfidation in the freshwater wetland mesocosms was slower than expected based on laboratory studies and that the sulfidation had occurred only partially after 18 months and leaching of Ag + was still possible. Once fully sulfidized, Ag 2 S is very stable as it does not oxidize during prolonged aeration (Choi et al 2009) and remains stable in wastewater sludge over more than 50 years (Donner et al 2015).…”

Silver sulfide nanoparticles (Ag2S-NPs) are taken up by plants and are phytotoxic

“…9,27 Similarly, synchrotron-based XAS analyses revealed that after ≥10 d wastewater treatment, the majority of Ag (more than 83% of the total Ag) was converted to Ag2S, regardless of the form in which Ag was added. 7,8,16 Finally, in a study using archived, stockpiled, and contemporary biosolids from the UK, USA, and Australia, Donner et al 28 reported that ≥64% of the total Ag was present as Ag2S in a wide range of materials, ranging from freshly-produced sludges to biosolids weathered under ambient environmental conditions for ≥50 years.…”

“…For example, after 400 d incubation, the Ca(NO3)2-extractable Ag concentration was 2.87 µg Ag kg -1 soil, only accounting for 0.29% of the total Ag in soil (Figure 3)this being in agreement with previous findings. 14,15,28 This low availability is unsurprising given the stability (i.e. low solubility) of Ag2S.…”