The mobility of two emerging contaminants, gallium and indium, in the soil – plant system

Abstract

Gallium (Ga) and indium (In) are non-essential trace elements present at low concentrations in Earth’s crust. They have a variety of applications, particularly in portable electronic devices such as mobile phones and portable computers. With increasing technological dependence, the risk of soil contamination is rising, through mining, extraction, and product disposal. Entry to the food chain could be detrimental to the health of humans and ecosystems. Relatively little is understood regarding behaviour in the environment, but both elements are believed to readily hydrolyse and precipitate in soil, with root retention further restricting movement into plant shoot biomass. This study aimed to determine the mobility of Ga and In in the soil-plant system. Batch sorption experiments were completed to quantify the degree of retention to the soil matrix. Perennial ryegrass (Lolium perenne L.) was grown in spiked soil to measure the capacity for uptake and translocation to above-ground biomass. Hydrolysis models were produced with a numerical equilibrium speciation model to estimate the predominant species in solution, providing an aid to understand of the processes occurring. Gallium and In were strongly retained by the soil matrix; distribution coefficients (Kd; measure of partitioning between solid and aqueous phases) averaged over a range of concentrations were approximately 400 and 2000 respectively. Mobility increased with pH (from pH 5.54-7.14, Ga Kd decreased 689-265 and In 1266-458), decreased with time, and relative mobility decreased with the concentration of Ga and In entering the system (Ga Kd increased 88-392 when application rose from 1-30 mg L-1, though solution concentrations increased from 0.06-1.33 mg L-1). Ga/In(OH)3 precipitation is believed to largely contribute to the sorbed fraction, and increased dominance of the anion (Ga(OH)4-) is likely responsible for higher Ga mobility. Low concentrations of both elements were present in shoot biomass; Ga reached 11.57 mg kg-1, with In plateauing at 0.015 mg kg-1 in the highest two treatments. Relatively little of the total mass applied to soil was harvested, peaking at 0.013% Ga and 0.0021% In. Even when the fraction retained to the soil matrix was estimated, there was little movement from solution to shoots, hypothesised to be attributed to sorption to outer root cells. Data showed that mobility of both elements were low, thus there is little risk of Ga and In contaminants entering the food chain via physiological uptake, though entry via other pathways such as atmospheric deposition (of aerosols, onto ingested foliage) cannot be ruled out. Strong retention in the upper soil profile is predicted. Future research is required to verify if these findings are applicable across a range of environmental conditions, along with the effect of other contaminants on soil mobility and plant uptake.