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Ecotoxicology and Human Environmental Health

The risk of silver transfer from soil to the food chain is low after long-term (20 years) field applications of sewage sludge Peng Wang, Neal W Menzies, Chen Hongping, Xinping Yang, Steve P. Mcgrath, Fangjie Zhao, and Peter M Kopittke Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00204 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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Environmental Science & Technology

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The risk of silver transfer from soil to the food chain is low after long-term

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(20 years) field applications of sewage sludge

3 4

Peng Wang1,*, Neal W. Menzies2, Hongping Chen1, Xinping Yang1, Steve P. McGrath3,

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Fang-Jie Zhao1, and Peter M. Kopittke2

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1

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Innovation Center for Solid Organic Waste Resource Utilization, College of Resources and

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Environmental Sciences, Nanjing Agricultural University, Nanjing, Jiangsu 210095, China

Jiangsu Provincial Key Lab for Organic Solid Waste Utilization, Jiangsu Collaborative

10

2

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Queensland 4072, Australia

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3

School of Agriculture and Food Sciences, The University of Queensland, St. Lucia,

Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, United Kingdom

13 14 15

*Corresponding author: Peng Wang, Phone: +86 25 8439 6509, Email: [email protected]

1 ACS Paragon Plus Environment


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Abstract

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The increasingly widespread usage of silver (Ag) nanoparticles has raised concerns regarding

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their environmental risk. The behavior of Ag and its transfer risk to the food chain were

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investigated using a long-term field experiment that commenced in 1942 in which Ag-

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containing sewage sludge was repeatedly applied to the soil (25 applications during 20 years).

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The speciation of the Ag in both the sludge and the soils retrieved from the long-term

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experimental archive was examined using synchrotron-based X-ray absorption spectroscopy,

23

and extractable Ag concentrations from soils determined using 0.01 M Ca(NO3)2 and 0.005 M

24

DTPA. The total Ag in the sludge during 1942 to 1961 ranged from 155 to 463 mg kg-1.

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These values are 1-2 orders of magnitude higher than those in currently-produced sludge (ca.

26

0.5-20 mg kg-1). Long-term repeated applications of these sludges resulted in an increase of

27

Ag in soils from 1.9 mg kg-1 in the control to up to 51 mg kg-1. The majority (> 80%) of the

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Ag in both the sludge and the sludge-treated soils was present as insoluble Ag2S, thereby

29

markedly reducing the bioavailability of this Ag. Concentrations of Ag in the archived crop

30

samples were generally < 0.70 mg kg-1 in edible tissues, much less than that those in diets that

31

may cause an adverse effects in animals and humans (>100 mg kg-1). These data indicate that

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the transfer of Ag (derived from both traditional Ag industry and current nano Ag industry) to

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the terrestrial food chain is limited.


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TOC

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INTRODUCTION

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Silver nanoparticles (Ag-NPs) exhibit strong and broad-spectrum antimicrobial properties,

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driving the development of Ag-NP products. Ag-NPs are used in numerous consumer

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products, ranging from detergents, textiles, and home appliances to socks, toothpaste, air

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filters, and nutritional supplements.1 The number of manufacturer-identified products

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containing Ag-NPs has increased substantially over the last decade. According to the nano-

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product database (accessed on 16 December 2017), there are 442 Ag-NP-containing products

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(from a total of 1827 nano-products) in the USA market1 and 378 (of 2586) in the European

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markets2, more than any other metal-containing nanomaterial product. Consequently, the

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release of Ag (including Ag-NPs and bulk Ag) into managed and natural ecosystems is

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increasing as a result of their production, utilization, and disposal. This has raised substantial

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concerns regarding the risks of Ag-NPs upon their release into the environment, also raising

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new challenges for environmental managers and policy-makers.

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Silver has a long history of usage in healthcare and medicine.3 During 1950-1990s,

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photography was the single largest user of Ag in industrial applications. For example, in 1995,

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photography consumed 6,527 tons of Ag, accounting for 28% of the total Ag demand (23,567

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tons).4 Due to the development of digital technology, demand for Ag in photographic films

55

has declined substantially; for instance, from 6,903 tons in 2000 to 1,446 tons in 2015.5 Most

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of the Ag used in traditional photography was subsequently discharged into the waste water

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treatment plants (WWTPs),6 which is also the main technical compartment through which the

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Ag-NPs are released into the environment.7-9 Once Ag enters into WWTPs, the majority (>

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80%) of Ag becomes sulfidated, irrespective of the forms in which it enters into WWTPs.10-12

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Therefore, we contend that it is possible that through studying the earlier long-term release of 4 ACS Paragon Plus Environment

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Ag from photographic usage, we could gain an understanding, at least partially, of the

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potential environmental risk of the current Ag-NP industry.

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The main pathway by which Ag enters into terrestrial ecosystems is via sewage sludge

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applications.13 Few studies have investigated the fate, stability, and (bio)availability of Ag in

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soils through AgNO3- or Ag-NP-preloaded sludge application or through the direct addition

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of Ag2S into soils. For instance, Pradas del Real et al. (2016) applied Ag-preloaded sludge to

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soils (at a 1/10 ratio of sludge to soil) in which wheat (Triticum aestivum) or oilseed rape

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(Brassica napus) was grown for four weeks in a pot experiment.14 They found that Ag bound

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with sulfur was the main species remaining in the sludge-treated soils, with no detectable

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translocation of Ag to the plant shoots. The solubility of Ag was very low when measured as

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Ag in pore water (below the detection of limit, < 1 µg L-1) or DTPA- and CaCl2-extractable

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Ag (less than 0.1% of the total Ag) in sludge-amended soils, even when the total Ag

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concentration was as high as 61 mg kg-1 soil. This result is consistent with the study of Wang

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et al. (2016) who showed that Ag2S was remarkably stable in sludge-treated soils, with the

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bioavailability of Ag being very low,11 and is also consistent with the study of Sekine et al.

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(2015) who found a very low lability of Ag in soils spiked with Ag2S-NPs.15 Similarly,

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Doolette et al. (2015) used a pot experiment to examine the bioavailability of Ag to lettuce

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(Lactuca sativa) in a soil amended with biosolids containing Ag2S-NPs, finding that plant

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uptake of Ag was low for all treatments.16 More recently, using a pot experiment, Wu et al.

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(2017) studied the accumulation of Ag in grains of rice (Oryza sativa) and wheat when grown

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in soils repeatedly amended with domestic or industrial biosolids (containing 0.64-7.5 mg Ag

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kg-1 for domestic biosolids and 1.1-9.3 mg Ag kg-1 for industrial biosolids). These authors

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found that after four annual applications, concentrations of Ag increased up to 20.8 µg kg-1 in 5 ACS Paragon Plus Environment


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whole wheat grain, but were not significantly different in brown rice compared to those in the

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control (without sludge application).17 These studies using either soil incubation or pot

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experiments provide important information regarding the fate and subsequent risk of Ag in

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the wastewater-sludge-soil pathway. However, little information is available regarding the

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stability and bioavailability of Ag from long-term field trials.

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In the present study, we examined the fate of Ag in soils and its subsequent transfer risk

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through the food chain based on the long-term sewage sludge application field experiment

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from Rothamsted Research (the Woburn Market Garden experiment, Woburn, Bedfordshire,

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England). The experiment started in 1942, with the sewage sludge being applied 25 times

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from 1942 to 1961. During this period, the concentrations of Ag in the sewage sludge ranged

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from 125 to 463 mg kg-1, reflecting the widespread usage of Ag in photography. The aims of

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the present study were to i) determine the speciation of Ag in sludge and in soils receiving

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sludge applications, (ii) examine the stability and availability of Ag in soils in long term (up

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to 23 years), and (iii) assess the bioaccumulation of Ag in edible and non-edible parts of

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crops , including spring barley (Hordeum vulgare), bean (Phaseolus vulgaris), cabbage

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(Brassica oleracea), carrot (Daucus carota), peas (Pisum sativum), potato (Solanum

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tuberosum), red beet (Beta vulgaris), and sugar beet (Beta vulgaris). These data not only

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provide information on the risk of Ag in terrestrial ecosystems associated with historical

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photographical usage of Ag, but also could establish a precedent for assessing whether the

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current rapidly-growing Ag-based nanotechnology constitutes an environmental risk.

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MATERIALS AND METHODS

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Field Experiment and sampling 6 ACS Paragon Plus Environment

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The field experiment (Market Garden Experiment, Rothamsted Research) was started in 1942

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at Woburn, Bedfordshire, UK (http://www.era.rothamsted.ac.uk/Other#SEC10). This field

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experiment was originally designed to investigate the effects of organic manures (including

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sewage sludge) on soil organic matter (SOM) and crop yield. The treatments included two

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rates of sewage sludge (S1 and S2), two rates of farmyard manure, two rates of sludge

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compost (a mixture of sewage sludge and straw), and a control (with inorganic fertilizers

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only). The soil was a sandy loam of Typic Udipsamment (U.S. soil taxonomy system). Basic

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information of this experiment has been provided previously.18, 19 The sewage sludge was

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applied 25 times between 1942 and 1961, with cumulative loadings of organic matter of 165 t

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ha-1 and 330 t ha-1 dry mater for the low (S1) and high (S2) application rates, respectively.

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The sewage sludge that was applied had been anaerobically digested and lagoon-dried, and

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was from the same sewage works in West London from 1942 to 1961. The sludge was later

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found to contain high concentrations of heavy metals, including Ag, which ranged from 155

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to 463 mg kg-1. In the late 1970s, concerns were expressed about the heavy metals being

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applied in the sludge. Subsequently, this field experiment was re-focused to examine the fate

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of metals that had been applied through sludge applications between 1941 and 1961.

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The field experiment was replicated in two side-by-side series, each containing four blocks

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with 40 plots (8.5 × 6.1 m) in a randomized design. The control treatment (i.e. inorganic

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fertilizers only) was replicated eight times in each series and the other organic manure

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treatments four times. From 1942 to 1973, spring barley and mainly vegetable crops including

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bean, cabbage, carrot, peas, potato, red beet, or sugar beet were grown in two reasons in

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rotation and in some years two crops were grown. Grass was grown from 1974 to 1982, and

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carrot and red beet were grown again from 1983 to 1985. White clover (Trifolium repens L) 7 ACS Paragon Plus Environment


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was grown from 1986 to 1989, and grass has been grown since 1989. Samples of sludge, soils,

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and plants are available from the Rothamsted archive.

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In the present study, soil and crop samples from the control and sludge application treatments

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were retrieved from the Rothamsted archive. The archived soil samples had been taken from

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the plow depth (23 cm) at irregular intervals in 1960, 1967, 1972, 1980, 1983, and 1984, air-

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dried, sieved to 2 mm and stored at room temperature in a dry environment. For the archived

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crop samples, carrot samples had been taken in 1963, 1984, and 1985; red beet in 1983, 1984,

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and 1985; sugar beet in 1970; and barley in 1971. All crops had both edible parts (roots or

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grain) and non-edible parts (tops or straw) and stored in a dry environment after oven-drying

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and grinding. Finally, 21 sewage sludge samples used from 1941 to 1961 were available and

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retrieved for analyses in the current study. For comparison, another 12 more recent sewage

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sludge samples (used in the Woburn long-term sludge experiments, Rothamsted Research)

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were also available and retrieved for Ag analyses. The 12 sludge samples were produced in

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1994 from five UK municipal WWTPs (Banbury, Coleshill, Perry Oaks, Carterton, and

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Selkirk).

149 150

Total and extractable silver in sludge, soils, and crops

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Total Ag concentrations were determined following digestion using aqua regia for soil and

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sludge samples20 and using HNO3/HClO4 (87/13, v/v) for crop samples (except for barley

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grain).21 Grain samples were digested in high-purity concentrated HNO3 with a microwave

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digestion system. Soluble and available Ag was extracted from soils with 0.01 M Ca(NO3)2

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(pH 6.0)11 or 0.005 M DTPA.22 Soils were weighed (0.5 ± 0.005 g) into 10 mL polypropylene

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centrifuge tubes, mixed with 5 mL extractant, shaken end-over-end for 2 h, and centrifuged at 8 ACS Paragon Plus Environment

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14,000 × g for 20 min. The supernatant was filtered (0.45 µm) and acidified with 70% HNO3

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for analysis. Silver concentrations in the digests or supernatants were determined using

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inductively coupled plasma mass spectrometry (ICP-MS, Perkin Elmer NexION 300X, USA).

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Quality control measures included the addition of indium as the internal standard and the use

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of procedural blanks, duplicates, and repeated analysis of certified references (including

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GBW_07428 for soils, NIST_1568b for rice, and GBW_10015 for spinach). Repeated

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analysis of the certified soil yielded Ag concentrations of 0.81 ± 0.013 mg kg-1, which are in

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good agreement with the certified value (0.084 ± 0.007 mg kg-1).

165 166

Silver speciation by X-ray Absorption Spectroscopy (XAS)

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Silver speciation was determined in situ for both sludge and soil samples using synchrotron-

168

based Ag K-edge X-ray absorption near edge structure (XANES) spectroscopy. We analyzed

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the sludge samples from 1944, 1950, 1955, 1961, and 1994, and soils from 1960, 1970, 1980,

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and 1984. Silver concentrations in sludges produced in 1994 were generally low, with the two

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highest sludge samples chosen for XAS analysis (denoted as 1994a and 1994b). Silver

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concentrations in crop samples were too low for XAS analysis. Soil and sludge samples were

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ground using a mortar and pestle, and sieved through a 250 µm sieve. The spectra were

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collected in a fluorescence mode with a 100-element solid-state Ge detector, at the XAS

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beamline of the Australian Synchrotron, Melbourne. A total of eleven Ag standard

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compounds were also examined, being metallic Ag, AgNO3, nano-Ag2S, bulk Ag2S, Ag2CO3,

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AgCl, Ag2O, Ag-cysteine, Ag-humic acid, Ag3PO4, and Ag-histidine.11 The averaged spectra

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of two to three scans for each sample were energy normalized using Athena software.23

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Principal component analysis (PCA) of the normalized spectra was used to estimate the likely

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number of species present in the samples, and target transformation (TT) was used to identify 9 ACS Paragon Plus Environment


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relevant standards for linear combination fitting (LCF).24 Both PCA and TT were undertaken

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using SixPack software25 and LCF analyses were performed using Athena software with a

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fitting range of -30 to + 100 eV relative to the Ag K-edge (25,514 eV) and a maximum of

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three standards was permitted for each fit. Given that the XANES spectrum of Ag2S NPs was

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indistinguishable from its bulk form, where LCF analyses indicated that Ag was present as

186

one of these two forms, their contributions were summed and defined simply as Ag2S.

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Statistical Analysis

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Data are presented as the mean ± standard error (SE) (n = 3-5). Treatment differences were

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tested for significance (p < 0.05) using a one-way analysis of variance (ANOVA) performed

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with IBM SPSS Statistics 20.

192 193

Results

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Concentrations and speciation of silver in sewage sludge

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Among the 21 sewage sludge samples archived at Rothamsted Research (UK) from 1944 to

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1961, concentrations of total Ag ranged from 125 to 463 mg kg-1 dry matter, with a mean of

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271 (± 19) mg kg-1 (Figure 1). These values are generally higher than those reported in more

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recent sludge samples from the UK. For example, concentrations of 5.6-131 mg kg-1 were

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reported from five municipal WWTPs in 1994, 3.4-18 mg kg-1 from six municipal WWTPs in

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200926 and 3.0-14 mg kg-1 from nine British WWTPs (with a median of 3.6 mg kg-1) in 2014

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(Figure 1).27 Similarly, total Ag concentrations in the USA were reported as being 246-332

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mg kg-1 in the 1970s and 15-27,000 mg kg-1 (mean of 4,612 mg kg-1 and median of 89 mg kg-

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1

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selected publicly owned WWTPs in 35 states in 2006-2007, total Ag concentrations ranged

) in the 1980s.26, 28 In a recent US national survey of sewage sludge from 74 randomly


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from 1.9 to 856 mg kg-1, with a mean of 31 mg kg-1 and a median of 13.6 mg kg-1.29 In

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Australia, Ag concentrations decreased from 35-74 mg kg-1 (with a mean of 56 mg kg-1) in

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the 1970s-1990s to 1.5-61 mg kg-1 (a mean of 10 mg kg-1) in 2009.26 Other studies have

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reported total Ag concentrations of 1.1-33 mg kg-1 from 50 sewage treatment works in

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Sweden in 2000,30 1-50 mg kg-1 in Germany in 1991,31 1.3-16 mg kg-1 in India in 2007,32 and

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0.6-7.5 mg kg-1 (with a mean of 3.0 mg kg-1) for 58 dewatered biosolid samples collected

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from WWTPs across China in 2013 (Figure 1).17 Generally, concentrations of Ag have

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decreased substantially over the recent decades, with the values now being 1-2 orders of

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magnitude lower those in the 1940-1950s (Figure 1).

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Sludge Ag (mg kg-1 dw)

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102

101

100

10-1 1940

UK USA Australia Sweden Switzerland

1950

Germany China Japan India Present study

1960

1970

1980

1990

2000

2010

2020

Year

214 215

Figure 1. Concentrations of total silver in sewage sludge measured in the present study and

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compiled from reported values in the literature. The Ag data were from sludge samples

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retrieved from the Rothamsted Research in this study and the reported values for the UK,26

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the USA,26, 28, 29 Australia,26, 33 Sweden,34 for Switzerland,13 Germany,31 India,32 Japan,35 and

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China.17 11 ACS Paragon Plus Environment


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The XANES spectra obtained for the archived sludge samples (between 1944 and 1994)

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visually resembled that of Ag2S reference (Figure 2a). Indeed, using LCF, it was predicted

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that > 80% of the Ag in these sludge samples was present as Ag2S, with the remaining being

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Ag3PO4 (5-11%) and Ag2O (0-8%) (Figure 2b). These results indicate that the majority of Ag

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in these older, archived sludge samples was present in a form similar to that found in more

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recent sludges.10-12, 14, 36 The Ag speciation in these archived sludge may potentially change

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over time during the storage, but Donner et al. (2015) has confirmed the sulfidation by

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analyses of Ag speciation in archived, stockpiled, and contemporary biosolids from the UK,

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the USA, and Australia over a period of more than 50 years.26 Furthermore, it should be

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noted that XAS is generally unable to identify chemical species that accounts for 80%, Figure 2) in the sludge and sludge-amended soils during the 23 years

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after sludge applications. As a consequence, the accumulation of Ag in crop plants was low

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(Figure 5), being substantially lower than the level that may cause an adverse effect in

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animals (≥ 100 mg Ag kg-1 in diets).38. These results indicate a limited risk of Ag transfer to

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the food chain.

325 326

The concentrations of Ag in the sludge applied from 1942 to 1961 were comparable to Ag

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concentrations reported in the other samples prior to the 1990s (Figure 1). During this period,

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the Ag in the sewage sludge was mainly derived from the photographic manufacturing and

329

processing.6 For example, photography was the single largest user of Ag in industrial

330

applications and consumed 6,527 tons in 1995, accounting for 28% of the total Ag demand

331

(23,567 tons).4 Due to the advent of digital technology, the usage of Ag in film photography

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has declined markedly, and as a result, concentrations of Ag in sludge have also decreased.

333

Indeed, the concentrations of Ag in the sludge (produced in 1942-1961) used in the present

334

study are almost 1-2 orders of magnitude higher than those in newly-produced sludge of the

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early 21st century (ca. 0.5-20 mg kg-1) (Figure 1). Given increasing demand for Ag-NP18 ACS Paragon Plus Environment

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containing consumer products and for other industrial processes (e.g. photovoltaics

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manufacture and ethylene oxide production),5 it is expected that Ag concentrations in sewage

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sludge are likely to increase again in the near future. Indeed, the Ag concentrations in the

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sludge derived from usage of Ag-NPs have been estimated to increase between 2005 and

340

2012,8 but the mean Ag concentrations in sludge from the Ag-NP sources are estimated as

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being < 2 mg kg-1.7, 8 Currently, Ag-NP production has been estimated at less than 1 kg in

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France,39 only 0.6-92 tons per year in the EU,40-42 2.8-20 tons in the USA,7, 42 and 5.5-320

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tons worldwide,34, 43-45 accounting for only 0.15-8‰ of the total demand for Ag (36,578 tons).

344

Therefore, the new applications of Ag-NPs are not expected to exceed the historical

345

contribution of photography to sludge Ag concentrations in the coming decades.

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The main pathway by which Ag enters soils is via the application of sewage sludge. In the

348

present study, applications of sludge resulted in a substantial accumulation of Ag in soils

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(Figure 3). For example, the high rate of sludge application between 1942 and 1961 increased

350

soil Ag concentrations from 1.9 mg kg-1 to a total of 51 mg kg-1. This value is comparable to

351

the reported mean Ag in sludge-treated soils (12 mg kg-1, ranging from 0.1-133 mg kg-1), but

352

much higher than the predicted total Ag in the EU soils in 2010 (0.8-1.7 mg kg-1) based on

353

the current silver usage.13, 46 Similarly, the production and usage of Ag-NPs have been

354

estimated to cause increases of both total Ag and Ag-NPs in soils. For example, the Ag-NP

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concentration was estimated to be 0.002 mg kg-1 in EU soils in 2014 and ca. 0.007 mg Ag-NP

356

kg-1 by 2020.8 Generally, the estimated Ag-NP concentrations in soils were 2-3 orders of

357

magnitude lower than total Ag derived from its nano forms in soils receiving sludge, whilst

358

the latter was ca. 2-3 orders of magnitude lower than reported total soil Ag concentrations.8, 13



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These estimations suggest that the contribution of Ag-NP industrial to soil Ag concentration

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are likely to be minor.

361 362

The majority of Ag in sludge samples and subsequently in the soils of the present study was

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present as Ag2S (Figures 2 and 4), in agreement with previous studies. For example, it has

364

been reported that of the Ag that accumulates in sludge, more than 80% of this silver is

365

sulfidated to Ag2S within the WWTPs, regardless of the form in which it is added.10-12, 14, 36,

366

47-49

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speciation in archived, stockpiled, and contemporary biosolids from the UK, the USA, and

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Australia; the majority of the Ag has been reported to be present as Ag2S among a wide range

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of biosolids despite some samples being produced as early as the 1950s, when the main

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source Ag in WWTPs was from photographic manufacturing and processing.6 These results

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indicate that sulfidation is the dominant process occurring within WWTPs, irrespective of the

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form of Ag originally entering this compartment (e.g. Ag-NPs, Ag salts, or Ag halides).

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Following the addition of Ag into the soils through sludges, sulfidated Ag remains the

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dominant species of Ag (Figure 4), consistent with previous studies.11, 14, 15 The presence of

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this insoluble form of Ag greatly reduces the bioavailability of Ag, as confirmed by the

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DTPA- and CaCl2-extraction (Figure S1) and accumulation of Ag in crop tissues (Figure 5

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and Figure S2). Generally, soluble or extracted Ag ranged from undetectable to 20 µg kg-1,

378

accounting for < 0.1% of total Ag in the soils in the present study (Figure S1) and previous

379

studies.14 Long-term application of Ag-containing sludge to soils increased tissue Ag in plants,

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with the measured values up to 0.70 mg kg-1 in crop edible tissues, although no obvious trend

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was observed in Ag concentrations between the treatments of low and high rates of sludge

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applications (Figure 5). These values are higher than those in edible tissues of plants grown in

Sulfidation has also been confirmed as the dominant process through analyses of Ag


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agricultural soils where sludge is not applied, e.g. < 0.0015 mg kg-1 dry weight for grains and

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cereal products, < 0.085 mg kg-1 for vegetables, and < 0.05 mg kg-1 for fruits,30, 50 but are

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comparable to those in edible tissues of plants grown in sludge-treated soils, e.g. up to 3.1 mg

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kg-1 for vegetables, 0.031 mg kg-1 for grain and cereal product, and 0.04 mg kg-1 for grass.16,

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17, 51

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sludge-treated soils (total soil Ag concentrations up to 150 mg kg-1), elevated Ag levels were

389

only observed in leaves of lettuce (0.02-3.1 mg kg-1) compared to those in the control (0.02-

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0.09 mg kg-1) with no adverse growth effect observed.51 The transfer factor (the ratio of tissue

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Ag to soil Ag) varied largely among these studies, with values generally being

Field Applications of Sewage Sludge - ACS Publications - American

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