Lithium, an emerging environmental contaminant, is mobile in the soil-plant system

Abstract

Lithium (Li) is the lightest of the alkali metals and reportedly present in soil at concentrations of 20-30 mg kg⁻¹. World demand of Li is increasing at 8% per annum, driven primarily through the use of Li-ion batteries. There is a lacuna of information on the behaviour of Li in the soil-plant system. I aimed to measure the concentrations of Li in New Zealand soils and pastures, the sorption of added Li to soil and the uptake of Li by food and fodder species. New Zealand soils and pasture Li concentrations were determined in a field trial. Batch experiments were used to measure the sorption of Li by the Templeton Silt Loam (TSL) at various pH values. I grew rye grass (Lolium perenne), beetroot (Beta vulgaris), broccoli (Brassica oleracea), carrot (Daucus carota), leek (Allium porrum), lettuce (Lactuca sativa), radish (Raphanus sativus), spinach (Spinacia oleracea), corn (Zea mays), tomato (Solanum lycopersicum), and courgette (also called zucchini - Cucurbita pepo) in the TSL limed to a pH of 6.2. Sunflower (Helianthus annuus) was in soils spiked with 0, 10 & 30 mg Li kg⁻¹. Mature plants were dissected and the portions were analysed separately. Lithium concentrations in soils tested from around New Zealand ranged from 0.08 mg kg⁻¹ to 92 mg kg⁻¹. The highest Li concentrations were found in soils with high clay content. Most endogenous Li in soil is insoluble and hence unavailable to plants. However, when exogenous Li is added to soil, there is only limited sorption of Li. Lithium sorption increased with increasing soil pH and decreased with increasing Li concentrations. Compared to other cations in soil, Li is mobile and may leach into receiving waters or be taken up by plants. When grown in uncontaminated soil, the Li concentrations in the edible portions of various plant uptake of Li differed by two orders of magnitude. Salt tolerant plants, namely B. vulgaris and S. oleracea took up the most Li, while seed and fruit crops had the lowest Li concentrations. Pasture grass (L. perenne) had the highest bioaccumulation factor for Li of any of the plants tested. When Li was added to soil, there were few differences in the uptake or tolerance of this element between species. At a soil concentration of just 5 mg kg⁻¹, the plants took up several hundred mg kg⁻¹ Li into the leaves with no reduction in biomass. At such high Li concentrations, only a small amount of plant material would need to be consumed to exceed the Tolerable Daily Intake for Li. 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. Therefore, planting fruit or seed crops on Li-contaminated soil may reduce the risk posed to human health. Future work should focus on the release of Li from electronic waste and industrial sources. Problems associated with Li mobility and plant uptake may be potentially relevant in tropical environments where informal e-waste reprocessing occurs.