Environmental Pollution 186 (2014) 195e202
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Assessing the influence of compost and biochar amendments on the mobility and toxicity of metals and arsenic in a naturally contaminated mine soil Luke Beesley a, *, Onyeka S. Inneh b, Gareth J. Norton b, Eduardo Moreno-Jimenez c, Tania Pardo d, Rafael Clemente d, Julian J.C. Dawson a a
The James Hutton Institute, Craigiebuckler, Aberdeen AB15 8QH, UK Institute of Biological and Environmental Sciences, University of Aberdeen, Aberdeen AB24 3UU, UK Universidad Autónoma de Madrid, 28049 Madrid, Spain d CEBAS-CSIC, PO Box 164, 30100 Espinardo, Murcia, Spain b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2013 Received in revised form 19 November 2013 Accepted 27 November 2013
Amending contaminated soils with organic wastes can influence trace element mobility and toxicity. Soluble concentrations of metals and arsenic were measured in pore water and aqueous soil extracts following the amendment of a heavily contaminated mine soil with compost and biochar (10% v:v) in a pot experiment. Speciation modelling and toxicity assays (Vibrio fischeri luminescence inhibition and Lolium perenne germination) were performed to discriminate mechanisms controlling metal mobility and assess toxicity risk thereafter. Biochar reduced free metal concentrations furthest but dissolved organic carbon primarily controlled metal mobility after compost amendment. Individually, both amendments induced considerable solubilisation of arsenic to pore water (>2500 mg l1) related to pH and soluble phosphate but combining amendments most effectively reduced toxicity due to simultaneous reductions in extractable metals and increases in soluble nutrients (P). Thus the measureemonitor-model approach taken determined that combining the amendments was most effective at mitigating attendant toxicity risk. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Soil contamination Organic amendments Trace elements Speciation Pore water
1. Introduction Contaminated, industrially impacted, mining and urban lands are not only characterised by young, poorly developed soils but often by their scarcity or absence of vegetation cover (Mench et al. 2010) associated with heavy metal toxicity. As well as restoring natural cycling of organic matter and nutrients, re-vegetation of contaminated soils is key to onward remediation. The presence of a vegetative cover over bare soil reduces the potential for migration of contaminants to proximal watercourses or inhalation following soil erosion and windblow (Tordoff et al., 2000; Arienzo et al., 2004; Ruttens et al., 2006) but a major limitation to re-vegetation is phyto-toxic concentrations of heavy metals in soils (Pulford and Watson, 2003). Organic soil amendments, such as composts, manures and sludges are now established amongst in-situ alternatives to expensive and/or disruptive hard-engineered removal or
* Corresponding author. E-mail addresses:
[email protected], (L. Beesley).
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0269-7491/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2013.11.026
capping of contaminated substrates to reduce contaminantassociated risk (Brown et al., 2003; Hartley et al., 2009). The contaminated site remediation agenda now relies more heavily on assisted natural attenuation measures, such as promotion of soil stability using retro-applied organic materials, increasingly viewed as both more environmentally harmonious and cost-effective than ex-situ works. Composts are produced by spontaneous microbial bio-oxidation of raw wastes to produce a biologically stable, humified organic matter end-product from, amongst myriad of other sources, green and agro-food industrial wastes (Bernal et al., 2007). In the latter category, ‘alperujo’, a waste derived from olive oil production, is abundantly available in Mediterranean regions and known for its fertilisation qualities (Fornes et al., 2009). Once conveniently composted, it is able to increase organic matter (OM), total-organic carbon (TOC), and microbial biomass C and N in soils, which stimulates plant growth on bare contaminated substrate (Clemente et al., 2012). These provisions may be particularly useful to old mine sites, typically existing with degraded or skeletal soils, depleted in organic matter and nutrients, but abundant with phyto-toxic metalliferous spoils (Wong, 2003). Other organic materials, such
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L. Beesley et al. / Environmental Pollution 186 (2014) 195e202
Fig. 1. a) Panoramic view of the wider soil sampling area, b) experimental pot set-up showing rhizon pore water samplers in-situ and c) petri-dishes for toxicity seed germination bio-assay in preparation.
as biochars, which are biomass pyrolysed under limited oxygen supply, have also gained favour recently in the same context due mainly to their ability to sorb metals, reducing phyto-toxic effects, which would otherwise be a barrier to initial re-vegetation of bare soils (Beesley et al., 2011; Gomez-Eyles et al., 2013). General benefits demonstrated by the experimental application of biochars to soils have been increased water holding capacities (Thies and Rillig, 2009), C, N and P status (Lehmann, 2007; Chan and Xu, 2009; Borchard et al., 2012), enhanced availability of Ca, Mg and Zn (Major et al., 2010; Gartler et al., 2013), but reductions in the leaching of some macronutrients in solution (Novak et al., 2009; Laird et al., 2010). In the context of pollution control, the removal of heavy metals and As from waste-waters (Mohan et al., 2007) and heavy metals from soil leachates (Beesley et al., 2010; Beesley and Marmiroli, 2011; Fellet et al., 2011) have also been reported as a consequence of biochar additions. Both alperujo composts and biochars have been proven to contain low concentrations of some metals and As, often below limits of detection, especially in the case of biochars (Clemente et al., 2012; Freddo et al., 2012). This means the risk of introducing extra contaminant load is minimal after their addition to soils. For example, As concentrations in redwood, maize, rice straw and bamboo biochars of
