Remediation of potentially toxic elements using live, nuisance macroalgae

Abstract

Rapid industrialisation over the last century has led to the rise of complex pollution issues in freshwater and marine environments. Pollutants from industrial, domestic, and municipal sources include antibiotics, nutrients, and, most notably, potentially toxic elements (PTEs). PTEs include heavy metals, light metals, and metalloids that pose dose-dependent toxicity risks following acute or chronic exposure. These elements may be released directly into aquatic environments through the discharge of contaminated wastewaters or indirectly through urban or agricultural runoff. Although at trace or diffuse concentrations these elements do not pose significant toxicity risks, they are persistent, accumulating in primary producers and magnifying through trophic levels in a process of biomagnification. The main actors for this initial accumulation in aquatic ecosystems are micro- and macroalgae. Algae are a ubiquitous, prolific and diverse class of primary producers that accumulate PTEs through passive and active mechanisms. This accumulation is achieved through essential resource scavenging mechanisms that improve trace micronutrient utilisation in algae.

Though PTE magnification poses ecological or environmental risks in the natural environment, it has distinguished algae as potentially powerful and sustainable wastewater treatment tools for the sorption and accumulation of PTEs.

Conventional wastewater treatment methods exist for managing the direct release of PTEs, however, these conventional methods are often expensive, energy-intensive, and inefficient at mg L-1 concentrations of PTEs. Algae may be able to mitigate these shortcomings by providing accessible and effective removal of PTEs.

This Ph.D. project assessed the efficacy of live Cladophora sp., C. parriaudii and C. coelothrix, to be used for sustainable PTE remediation. These algae species were selected as they are able to flourish in eutrophied environments and have been observed to accumulate a range of essential and nonessential PTEs. Four PTEs were investigated during this study, i.e. Se, Cu, Mn, and V, and were selected based on their prevalence in the environment, industrial wastewaters, or their economic value if recovered. To provide a full suitability and sustainability assessment of these algal sorbents, experiments were conducted to identify fundamental tolerance, optimal exposure period for PTE removal, PTE accumulation mechanism, and biomass reuse potential.

The first set of experiments summarised fundamental information on PTE toxicity and removal displayed by each Cladophora species following prolonged PTE exposure. Several concentrations were tested (0.0, 1.0. 2.0, 5.0, 10, and 20 mg L-1 for Se, Mn and V; 0.0, 0.1, 0.25, 0.5, 0.75, and 1.0 mg L-1 for Cu). The Cladophora species tolerance to these PTEs was determined using several metrics as proxies for productivity. These proxies included nutrient removal, pH change, pigmentation, and biomass yield. Whilst, PTE removal was quantified by analysing differences in initial and final PTE concentrations (Day 0 and 14, respectively). Both Cladophora species were able to maintain suitable productivity across all tested PTEs and PTE concentrations. Accumulation varied by species, PTE, and PTE dose. Notably, neither species achieved significant accumulation of Mn or V. C. parriaudii cultures accumulation varied from 0.11 – 2.75 mg g-1 DW and 0.062 – 0.58 mg g-1 DW for Se and Cu, respectively depending on PTE dose. These values corresponded to removal efficiency of 5.8 – 31 % and 15 – 42 % for Se and Cu, respectively. C. coelothrix also achieved accumulation between 0.51 – 2.5 mg g-1 DW and 0.071 – 0.51 mg g-1 DW for Se and Cu, respectively. These values corresponded to removal efficiency of 9.9 – 16 % and 16 – 44 % for Se and Cu, respectively. Both species achieved peak Se and Cu uptake at 5.0 mg Se L-1 and 1.0 mg Cu L-1.

Detailed time-course studies were next used to identify the optimal level of PTE exposure and the potential PTE sorption mechanism and location. To identify optimal exposure, frequent media sampling was used to quantify PTE removal during the exposure period (0.5 h, 1 h, 6 h, 24 h, 3 days (72 h), 7 days (168 h), and 14 days (336 h)). PTE sorption mechanism was estimated using a combination of adsorption kinetics modelling, Fourier Transform Infrared Spectrum, and Scanning Electron Microscopy. There were species- and PTE-specific differences in accumulation rate and potential sorption mechanism. C. parriaudii reached peak Se accumulation at the day 7 (168 h) sampling time, reaching 4.35 mg Se g-1 DW, and achieved peak Cu accumulation at the day 14 (336 h) sampling time (0.83 mg Cu g-1 DW). C. coelothrix reached peak Se and Cu accumulation at the day 14 (336 h) sampling time, reaching 2.45 mg Se g-1 DW and 0.93 mg Cu g-1 DW. In terms of potential sorption mechanisms, the tested species displayed different rates of removal depending on PTE, and displayed changes to FTIR spectra and SEM images that corresponded to these differences.

Finally, a feasibility study was used to assess the reuse potential of each Cladophora species following PTE exposure. Specifically, three bioenergy conversion routes were assessed. Hydrothermal liquefaction, anaerobic digestion, and fermentation were selected based on literature review of bioenergy production options. Suitability, in terms of magnitude of fuel produced and cost of production, of each species was considered. Biochemical composition through the PTE exposure period helped to inform optimal exposure length for suitable biochemical status and theoretical estimates for bioenergy fuels. The PTE-loaded Cladophora species reported theoretical fuel production similar to other tested macroalgal feedstocks. Additionally, feedstock dosing optimisations were identified to mitigate complications associated with the high PTE content feedstock.

Ultimately, this piece of work provided a full assessment of the application potential of each live algal sorbent quantifying fundamental tolerance and PTE removal, characterising PTE sorption mechanisms, and identifying opportunities and barriers to biomass reuse options. This research also identified a lack of standardisation in experimental methods within the biosorption literature, and highlighted opportunities for further research into pilot-scale application of algal sorbent technologies.