Development and evaluation of graphene-based adsorbents for remediation of soil contaminants

Abstract

Contaminated soils contain a mix of different contaminant-types; efficient simultaneous in situ remediation is challenging as a single process may not suffice. Adsorption is a favourable in situ technique. While graphene-based materials (GBMs) have recently been developed as adsorbents for contaminant-removal from water due to their unique functional properties, virtually no studies have investigated their potential in soil. This thesis investigates two prepared GBMs – graphene oxide (GO), and an iron-oxide-modified reduced-GO composite (FeG) – for simultaneous adsorption of 4 model contaminants – arsenate (As; an anionic metalloid), cadmium (Cd; a cationic metal), perfluorooctanoic acid (PFOA) and perfluorooctane sulphonate (PFOS). A ‘mixed’ mineral and carbon-based adsorbent, RemBindTM (RemB) was also tested for comparison. Positively-charged FeG showed a strong affinity for binding anionic As, whereas negatively-charged GO showed a strong affinity for binding cationic Cd. An increase in pH promoted Cd sorption and decreased As sorption. Arsenate sorption by FeG was comparable to that by RemB. GO displayed excellent Cd sorption even in acidic conditions, outperforming RemB. Competition by phosphate did not affect As sorption, whereas competition by Ca strongly suppressed Cd sorption. In the case of FeG and RemB, As binding was attributed to ligand-exchange mechanisms with hydroxyl groups on the mineral phases (goethite and alumina, respectively) of the adsorbents. Electrostatic interactions were identified as the main mechanism for Cd sorption by GO and RemB. A mixture of GO and FeG was successful in simultaneous sorption of Cd and As from co-contaminated solutions; amounts sorbed by this mixture were greater than that sorbed by RemB. Sorption of PFOA by FeG and RemB was much greater than GO. While sorption by GO was hindered at increased pH due to increased repulsion of the PFOA anion, sorption by FeG and RemB were unaffected by variations in pH and ionic strength. In addition to hydrophobic interactions with the carbonaceous phases, the role of combined Fe- and Al-mineral phases in FeG and RemB proved strategic in binding PFOA via multiple mechanisms. From an environmental partitioning perspective, precipitation from rainfall events is unlikely to desorb PFOA bound by FeG and RemB. However, leaching of bound PFOA is likely in the presence of polar organic solvent waste at waste disposal or landfill sites. The ‘mixed’ adsorbents, FeG and RemB, successfully sorbed a range of per- and polyfluoroalkyl substances (PFASs) from a contaminated field sample, demonstrating great potential for use in soil. During experimental work with 14C-PFOA, sorption losses of the analyte onto common laboratory ware were observed. Losses observed on polypropylene tubes were remarkably higher than on glass, contradictory to the published literature. Filtration was also determined to be a major source of error, leading to an underestimation of dissolved concentrations. These losses drew attention towards potential analytical bias related to PFASs during routine procedures. Finally, to test the remediation efficiency of GBMs in situ in a soil matrix, using singly-contaminated soils and a ‘cocktail’-contaminated soil containing As, Cd, PFOA and PFOS, impacts on contaminant bioaccessibility and microbial soil nitrification were measured. FeG and RemB greatly reduced bioaccessibility of As, PFOA and PFOS (but not Cd) by 89 – 100%, compared to GO (36 – 86%). The mixed-mineral and carbonaceous nature of FeG and RemB offered multiple binding pathways – i.e. hydrophobic interactions with the graphitic plane (for PFOA and PFOS), and ligand-exchange with the goethite or alumina phase (for As, PFOA and PFOS), for FeG and RemB, respectively. Despite the widely-demonstrated success of GO for Cd-removal from water, GO did not bind Cd in the soils. In fact, GO increased Cd-bioaccessibility by 2 fold compared to the unremediated control due to lowered pH (3.5) and concurrent release of calcium ions (Ca2+), which competed with Cd2+ for GO’s binding sites. Addition of GBMs severely impaired microbial-driven soil nitrification processes (55 – 99% inhibition) due to soil-acidification. While GBMs (particularly FeG) show great promise for reducing bioaccessibility of contaminant-mixtures, their potential to be used for effective in situ soil remediation requires that the acidity generated by the materials is neutralised. In summary, adsorbents (particularly, FeG and RemB) that provided multiple pathways for binding contaminants showed great potential for use as in situ soil adsorbents for simultaneous remediation of multiple contaminant-types. For GBMs to be applied efficiently in situ, the risk of soil acidification will require management.