E-waste comprises discarded electronic appliances, of which computers and mobile telephones are disproportionately abundant because of their short lifespan. The current global production of E-waste ...is estimated to be 20–25 million tonnes per year, with most E-waste being produced in Europe, the United States and Australasia. China, Eastern Europe and Latin America will become major E-waste producers in the next ten years. Miniaturisation and the development of more efficient cloud computing networks, where computing services are delivered over the internet from remote locations, may offset the increase in E-waste production from global economic growth and the development of pervasive new technologies. E-waste contains valuable metals (Cu, platinum group) as well as potential environmental contaminants, especially Pb, Sb, Hg, Cd, Ni, polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs). Burning E-waste may generate dioxins, furans, polycyclic aromatic hydrocarbons (PAHs), polyhalogenated aromatic hydrocarbons (PHAHs), and hydrogen chloride. The chemical composition of E-waste changes with the development of new technologies and pressure from environmental organisations on electronics companies to find alternatives to environmentally damaging materials. Most E-waste is disposed in landfills. Effective reprocessing technology, which recovers the valuable materials with minimal environmental impact, is expensive. Consequently, although illegal under the Basel Convention, rich countries export an unknown quantity of E-waste to poor countries, where recycling techniques include burning and dissolution in strong acids with few measures to protect human health and the environment. Such reprocessing initially results in extreme localised contamination followed by migration of the contaminants into receiving waters and food chains. E-waste workers suffer negative health effects through skin contact and inhalation, while the wider community are exposed to the contaminants through smoke, dust, drinking water and food. There is evidence that E-waste associated contaminants may be present in some agricultural or manufactured products for export.
Dissolved organic matter (DOM) release from Cd contaminated soils been linked to mobilisation of the metal as Cd-DOM complexes and this may be exacerbated by organic matter-rich soil amendments. The ...quantity and quality of the DOM can determine the proportion of dissolved Cd that partitions to mobile complexes and their stability and, thus, the potential for Cd transport from contaminated soils. The aim of this work was to examine differences in Cd mobilisation from soils to which different types of soil amendments/conditioners have been applied and the importance of DOM characteristics in determining the extent to which this can happen. Three soils were spiked with Cd to 2 mg kg−1, allowed to equilibrate and then treated with compost and peat. These soils and an untreated subsample of each soil were then adjusted to three different pHs: 5.6, 6.4 and 7.4, using lime. The amount of Cd mobilised from each soil was tested using a column leaching experiment. Ultrafiltration and speciation modelling were used to determine amounts of Cd as DOM-complexed, “truly” dissolved (<5 kDa) and colloidal species, while DOM quality was assessed using UV–Vis and fluorescence spectroscopy. Most colloidal Cd was mobilised from the compost treated soils (50%–60%), followed by the peat treated soils (20–44%). The relationships between colloidal Cd, DOC concentration and soil pH, together with the spectroscopic and modelling results showed that structural properties of DOM are an important factor in mobilising Cd from contaminated soils.
•Cd mobilisation from soils treated with organic matter-rich substrates was tested.•Mobilisation was reduced by the treatments at soil pH 5.6 and by liming to 7.4•Treating soils with compost and peat at pH 6.4 increased Cd mobilisation.•Dissolved organic matter quality can determine extent of Cd mobilisation.
Biochars are biological residues combusted under low oxygen conditions, resulting in a porous, low density carbon rich material. Their large surface areas and cation exchange capacities, determined ...to a large extent by source materials and pyrolysis temperatures, enables enhanced sorption of both organic and inorganic contaminants to their surfaces, reducing pollutant mobility when amending contaminated soils. Liming effects or release of carbon into soil solution may increase arsenic mobility, whilst low capital but enhanced retention of plant nutrients can restrict revegetation on degraded soils amended only with biochars; the combination of composts, manures and other amendments with biochars could be their most effective deployment to soils requiring stabilisation by revegetation. Specific mechanisms of contaminant-biochar retention and release over time and the environmental impact of biochar amendments on soil organisms remain somewhat unclear but must be investigated to ensure that the management of environmental pollution coincides with ecological sustainability.
► Biochars can reduce mobilities of some organic and inorganic pollutants in soil. ► Source material and production conditions influence pollutant retention. ► Highly alkaline pH and water soluble carbon can undesirably mobilise some elements. ► Large surface area may be toxic to soil fauna but create microbial niches. ► Efficacy of biochar may depend on other organic materials applied in combination.
Biochars can reduce the mobility and impact of some soil pollutants but, if applied alone, may fail to support soil restoration, revegetation and hence ecologically circumspect remediation.
Chelant-enhanced phytoextraction has received a lot of attention in the past decade. In theory, this technique could cleanse metal polluted soils by solubilizing contaminating metals, allowing them ...to be taken up by plants that would subsequently be removed from the site. We review the processes of metal solubilization, uptake by plants, and leaching during chelant-enhanced phytoextraction. A large excess of chelant is required to solubilize the target metal due to the co-solubilization of Ca and Fe. Chelated metals are taken up via the apoplastic pathway. Disruption of the Casparian Band is required to achieve the high shoot concentrations needed for phytoextraction. Therefore, adding chelants to a soil increases not only the total dissolved metal concentration but also changes the primary route of plant metal-uptake from the symplastic to the apoplastic pathway. Depending on metal, plant species, and chelant concentration, significant increases in metal uptake are likely. Soil solution chelate concentrations of at least several mM are required to induce appreciable shoot concentrations. A simple calculation reveals that at such soil solution concentrations plants will remove only a small fraction of the solubilized metals. Leaching, exacerbated by preferential flow processes, is unavoidable. Chelant-enhanced phytoextraction is therefore limited to areas where the connection with groundwater has been broken, or where leaching is unimportant. Chelant-enhanced phytoextraction may nonetheless have a role in enhancing the uptake of essential trace metals. Such a role warrants further investigations into the use of biodegradable chelants such as ethylenediaminedisuccinic acid (EDDS).
The efficacy of using plants to phytoremediate heavy metal (HM) contaminated soils can be improved using soil amendments. These amendments may both increase plant biomasses and HMs uptake. We aimed ...to determine the composite effect of ammonium sulfate ((NH4)2SO4) combined with the application of an aqueous stem-extracted bio-chelator (Bidens tripartita L) on the plant biomasses and cadmium (Cd) phytoextraction by Solanum nigrum L. The constant (NH4)2SO4 application mode plus bio-chelator additives collectively enhanced the shoot Cd extraction ability owing to the increased plant biomass and shoot Cd concentration by S. nigrum. The shoot Cd extraction and the soil Cd decreased concentration confirmed the optimal Cd phytoextraction pattern in K8 and K9 treatments (co-application of (NH4)2SO4 and twofold/threefold bio-chelators). Accordingly, Cd contamination risk in the soil (2 mg kg−1) could be completely eradicated (<0.2 mg kg−1) after three rounds of phytoremediation by S.nigrum based on K8 and K9 treatments through calculating soil Cd depletion. The microorganism counts and enzyme activities in rhizosphere soils at treatments with the combined soil additives apparently advanced. In general, co-application mode of (NH4)2SO4 and aqueous bio-chelator was likely to be a perfect substitute for conventional scavenger agents on account of its environmental friendliness and cost saving for field Cd contamination phytoremediation by S. nigrum.
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•◆Co-application of (NH4)2SO4 and bio-chelator was more effective than alone.•◆K8 and K9 treatments achieved the optimal Cd phytoextraction pattern.•◆The microorganism counts and enzyme activities advanced most for K8 and K9.•◆2 mg kg−1 Cd pollution could be removed after three rounds of phytoremediation.
Contamination of soil with lithium (Li) is likely to increase due to its wider dispersal in the environment, associated in particular, with the disposal of the now ubiquitous Li-ion batteries. There ...is, however, a paucity of information on the behaviour of Li in the soil-plant system. We measured the sorption of added Li to soil, and uptake of Li by food and fodder species. Around New Zealand, soil concentrations were shown to range from 0.08 mg/kg to 92 mg/kg, and to be positively correlated with clay content. Most geogenic Li in soil is insoluble and hence unavailable to plants but, when Li+ is added to soil, there is only limited sorption of Li. We found that Li sorption increased with increasing soil pH, and decreased proportionately with increasing Li concentrations. Compared to other cations in soil, Li is mobile and may leach into receiving waters, be taken up by plants, or have other biological impacts. In a soil spiked with just 5 mg/kg, plants took up several hundred mg/kg Li into leaves with no reduction in biomass. Lithium appears to be a phloem immobile element, with the highest concentrations occurring in the older leaves and the lowest concentrations occurring in the seeds or fruits. These results may raise concerns and risks in situations where food and fodder crops are associated with waste disposal.
•Geogenic Li is sparingly soluble and mostly unavailable to plants.•Ionic Li added to soil binds weakly and is more mobile under acidic conditions.•Plants take up ionic Li with a bioaccumulation coefficient >5.•Plants accumulate up to 500 mg/kg Li in the water sinks with few toxicity symptoms.•In contaminated environments, plants are likely to be an important exposure pathway for Li to enter humans or ecosystems.
•Biochar application decreased the concentration of extractable Cd, Cu, Pb and Zn.•Bamboo biochar was more effective than rice straw biochar in reducing the bioavailability of Cd.•Straw biochar was ...more effective than bamboo biochar in reducing the bioavailability of Cu, Pb and Zn.•Fine biochar was more effective than coarse biochar in reducing Zn concentration in plant shoots.
Soil contamination with heavy metals has become a global concern because of its adverse effects on ecosystem health and food security. Soil amendments including biochar can reduce the bioavailability of heavy metals in contaminated soils and reduce their risk of entering the food chain. A pot experiment was conducted to investigate the effects of biochars derived from bamboo and rice straw on bioavailability and plant growth in a sandy loam paddy soil naturally co-contaminated with Cd, Cu, Pb and Zn. The soil was moderately acidic (pH=5.7) and low in organic carbon content (8.7gkg−1). Bamboo and rice straw biochars, pyrolyzed at temperatures≥500°C and with two mesh sizes (< 0.25mm and<1mm), were applied at three rates (0, 1% and 5%, w/w). A metal-tolerant plant, Sedum plumbizincicola X. H. Guo et S. B. Zhou sp. nov. was used in the plant growth experiment to examine the bioavailability of these metals. The addition of biochars to soil significantly (p<0.05) increased the above-ground biomass of S. plumbizincicola. By the end of the experiment, soils amended with biochar had pH values significantly (p<0.05) higher, this effect being more accentuated at the high biochar dose and small particle size. The solubility of Cd, Cu, Pb, and Zn as measured by Toxicity Characteristic Leaching Procedure (TCLP) was significantly lower (p<0.05) in the biochar-amended soils than in the control soil. This was paralleled by significant reductions in Cd, Cu, Pb and Zn accumulated in the above-plant biomass of amended soils. Rice straw biochar reduced the concentration of Cu and Pb in the shoots by 46 and 71%, while bamboo biochar reduced concentration of Cd in the shoot by 49%. Finer biochar was more effective on reducing the concentrations of Zn in shoot than the coarse ones, while particle size had no effect on the concentrations of Cd, Cu and Pb in the shoot of S. plumbizincicola. In conclusion, the influence of biochar on heavy metal bioavailability varied not only with the feedstock and application rate of biochars, but also with the metal species. Therefore, biochar should be carefully designed to maximize the reduction of the bioavailability of a given heavy metal in soil.
Gallium (Ga) and indium (In) are increasingly susceptible to soil contamination via disposal of electronic equipment. Chemically similar to aluminium (Al), these elements may be mobile and ...bioavailable under acidic conditions. We sought to determine extent and nature of Ga and In mobility in the soil – plant system and thus their potential to enter the food chain. Batch sorption experiments on a high fertility silt loam (pH 5.95, CEC 22 meq 100 g−1) showed strong retention of both elements to the soil matrix, with mean distribution coefficient (KD) values of 408 and 2021 L kg−1 for Ga and In respectively. KD increased with concentration, which we attributed to precipitation of excess ions as insoluble hydroxides. KD decreased with increased pH as Ga/In(OH)2+ and Ga/In(OH)2+ transitioned to Ga/In(OH)4-. Movement into the aboveground portions of perennial ryegrass (Lolium perenne L.) was low, with bioaccumulation factors of 0.0037 for Ga and 0.0002 for In; foliar concentrations peaked at 11.6 mg kg−1 and 0.015 mg kg−1 respectively. The mobility of Ga and In in the soil - plant system is low compared to other common trace element contaminants such as cadmium, copper, and zinc. Therefore, Ga and In are likely to accumulate in soils and soil ingestion, either directly, via inhaled dust, or dust attached to food, will be the largest pathway into the food chain. Future work should focus on the effect of redox conditions on Ga and In, as well as uptake into acidophilic plants such as Camellia spp., which accumulate Al.
•Gallium and indium are more strongly bound to soil than other common trace element contaminants.•Gallium and indium are poorly translocated into the aerial portions of Lolium perenne.•Gallium and indium are likely to accumulate in soil.•Soil ingestion is the most likely exposure pathway for humans and animals.
The accumulation of Cd in soils worldwide has increased the demand for methods to reduce the metal's plant bioavailability. Organic matter rich soil amendments have been shown to be effective in ...achieving this. However, it is not known how long these amendments can retain the Cd, and whether dissolved organic matter (DOM) released from them can enhance the metal's mobility in the environment. In this study we sought to test the Cd binding capacity of various organic soil amendments, and evaluate differences in characteristics of the DOM released to see if they can explain the lability of the Cd-DOM complexes. We collected ten organic soil amendments from around New Zealand: five different composts, biosolids from two sources, two types of peat and spent coffee grounds. We characterised the amendments' elemental composition and their ability to bind the Cd. We then selected two composts and two peats for further tests, where we measured the sorption of Ni or Zn by the amendments. We analysed the quality of the extracted DOM from the four amendments using 3D Excitation Emission Matrix analysis, and tested the lability of the metal-DOM complexes using an adapted diffusive gradients in thin-films (DGT) method. We found that composts bound the most Cd and that the emergent Cd-DOM complexes were less labile than those from the peats. Ni-DOM complexes were the least labile. The aromaticity of the extracted DOM appears to be an important factor in determining the lability of Ni complexes, but less so for Zn and Cd.
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•Adapted DGT method measures complex lability in small volumes.•Composts bind Cd effectively, but may mobilize trace metals via DOM-complexes.•Nickel complexes are less labile than Cd or Zn complexes in compost extracts.•Aromatic DOM reduce lability of Ni complexes, and Zn to a lesser extent.
The relative ease with which cadmium (Cd) in agricultural soils can transfer to crop plants can pose a potential health risk to consumers. However, efforts to predict and mitigate these risks are ...often confounded by the various factors that influence metal accumulation in the edible plant parts. The aim of this work was to identify key drivers that determine Cd concentrations in spinach leaves, potato tubers, onion bulbs and wheat grain grown in commercial horticultural operations across New Zealand (NZ). Paired soil and plant samples (n = 147) were collected from farms across different NZ growing regions. Cadmium concentrations in the edible parts were measured and four different tests were used to examine the potential bioavailability of soil Cd: pseudo-total and porewater concentrations, 0.05 M Ca(NO3)2-extraction and diffusive gradients in thin-films (DGT). Information on a range of soil and climatic variables was also collected. The methods' ability to represent Cd concentrations in the plant parts was assessed through single and multiple regression analysis that considered the different variables and the farm locations. Soil Cd concentrations determined by the different tests were positively related to plant concentrations and there were clear regional differences between these relationships. The Ca(NO3)2 extraction predicted over 76% of the variability in Cd concentrations in onion bulbs and spinach leaves, while DGT and porewater Cd provided the best estimates for potato tubers and wheat grains, respectively, once regional differences were considered, along with certain environmental and soil variables. The results show that certain soil and environmental factors can be a key influence for determining Cd accumulation in the edible parts of some plants and that regional differences are important for modulating the extent to which this occurs. These effects should be considered when trying to mitigate the potential risks arising from Cd in agricultural soils.
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•Cadmium accumulation in soils increases risk of toxic concentrations in plants.•Soil Cd bioavailability was tested and compared to concentrations in crop plants.•0.05 M Ca(NO3)2, DGT and porewater tests were better predictors than total Cd.•Soil and environmental factors were important for determining Cd accumulation.•Understanding regional differences in Cd uptake is important for managing risk from soil Cd.
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