A comparative assessment of lesser-studied trace elements in the soil-plant system : implications for environmental quality. (2022)
Type of ContentTheses / Dissertations
Thesis DisciplineEnvironmental Sciences
Degree NameDoctor of Philosophy
The study of the chemical elements in the environment has focused on common, easily measured nutrients and contaminants that commonly affect humans and ecosystems. Technological development has resulted in the increased use of chemical elements that have received scant scientific attention as environmental contaminants. These Emerging Trace Element Contaminants (ETECs), include (in order of atomic number) beryllium (Be), gallium (Ga), indium (In), tellurium (Te), lanthanum (La), cerium (Ce), neodymium (Nd), gadolinium (Gd), and bismuth (Bi). Similarly, studies on iodine (I) have been limited due to analytical challenges.
The behaviour of ETECs and I in the soil – plant system is critical to understanding how these elements may affect humans and ecosystems. This thesis aimed to quantify the comparative behaviour of the ETECs and I in the soil-plant system, and determine the factors affecting their mobility. The ETECs were compared to cadmium (Cd), a well-studied relatively mobile contaminant. When the capacity of the ETECs and Cd to sorb to soil was measured in spiked soil, solid-solution distribution coefficients (KD) increased: I<Cd<Be<In<La<Ce<Nd<Gd<Ga, from <1 (I) to >800 (Ga). Bismuth and Te formed insoluble precipitates in solution thus had higher KD than the other ETECs and Cd, although their values could not be quantified. KD was highest for the trivalent cations, and within the same valency, KD increased with ionic potential. The KD values of Be, Cd, La, Ce, Nd, and Gd increased exponentially with increasing pH, which is typical for cations. In contrast the KD of Ga increased from pH 4.5-6 then decreased at pH >6. The pH had little effect on the KD of In. As the concentration of the ETECs added to soil increased, the KD of Be, La, Ce, Nd, and Gd decreased due to saturation of sorption sites, however, the KD of Ga and In increased due to precipitation. Langmuir and Freundlich sorption isotherms described the soil: solution portioning of Be, Cd, La, Ce, Nd, and Gd at concentrations <100 mg L-1 in ambient solution. However, the fit was poor at 100 mg L-1 TE added to solution, and for Ga and In.
Plant uptake was determined using experiments with perennial ryegrass (Lolium perenne) in pot trials using spiked soil that had been left to equilibrate. Bioaccumulation coefficients (BACs) increased in the order Ce<In<Nd≅Gd<La≅Be≅Ga<Cd<I. Due to plant uptake and the phytotoxicity thresholds, Be and La are the ETECs likely to be taken up and translocated into above-ground biomass at high concentrations. Analyses of Camellia sinensis (common tea, an aluminium hyperaccumulator) leaves revealed concentrations of In, La, Ce, Nd, and Gd that were higher than concentrations in Camellia sinensis than in L. perenne growing in unspiked soil, but less (except for In) that L. perenne growing in soil spiked with 20x the background concentration. Analysis of the concentrations of I, in New Zealand pastures revealed that distance to the coast had a stronger correlation with pasture I concentrations than soil I concentrations, due to the sea being a source of I to plants and soil. As a monovalent anion, KD of I in soil was less than the ETECs and Cd, and bioaccumulation factors (BACs) in perennial ryegrass were higher than the ETECs and Cd.
For elements with atomic numbers 13 (Al) – 49 (In), there was a significant negative correlation between their respective KD values and the Me-O bond length in the hydrated ions. Therefore, Me-O bond length could be used to calculate KD and of ETECs in uncontaminated soils. A similar negative correlation existed for the Rare Earth Elements. As KD was inversely proportional to BAC, the Me-O bond length could also indicate plant uptake.
Due to their strong sorption to soil and limited leaching and plant uptake, the most likely exposure pathway for Be, Ga, In, La, Ce, Nd, and Gd to enter humans is through the ingestion of contaminated soil, either directly (pica children, dust) or indirectly (soil particles attached to plants). Sites contaminated with ETECs should be managed to reduce soil erosion and thus reduce the fluxes of these elements. Future work should quantify interspecific differences in plant uptake of ETECs, analyse the distribution of the ETECs in plant material surrounding contaminated locations, and determine the bioaccessability of ETECs to humans and other organisms.
RightsAll Rights Reserved
Showing items related by title, author, creator and subject.
Campbell MH; Marek L; Curl A; Kingham S (2017)
Trace elements in three soil monosequences and iron-manganese concretions from Westland and Canterbury, New Zealand Dawson, Bernard S. W. (University of Canterbury. Chemistry, 1982)This work is a study of the first row transition metals Ti, V, Cr, M, Fe, Co, Ni, Cu and Zn in three soil monosequences and iron-manganese concretions. Three wet chemical decomposition techniques, to be used for the ...
Ji M; Greening C; Carere CR; Bay S; Steen J; Montgomery K; Lines T; Beardall J; van Dorst J; Snape I; Stott MB; Hugenholtz P; Ferrari B; Vanwonterghem, Inka (2017)Cultivation-independent surveys have shown that the desert soils of Antarctica harbour surprisingly rich microbial communities. Given that phototroph abundance varies across these Antarctic soils, an enduring question is ...