Removing metal contaminants from wastewater can be challenging, but laboratory studies have shown promising results using different algal species. The biomass used can itself be a waste product from industrial processes or an abundant marine species that may cause problems in some situations, such as algal blooms. Algae can uptake, immobilise, and totally or partially remove or transform toxic species through biosorption, bioaccumulation, bioconversion, biodegradation and biocatalysis. Biosorption is one of the most common methods used, as it offers high uptake efficiency, selectivity and biomass reusability at low cost. Previous studies showed that brown algae species have better adsorption capabilities than green and red species, likely due to the high polysaccharide content within their tissue and cell walls. However, for these bio sorbents to move from the lab to the real world, we need to understand the maximum uptake capacity for each metal and the mechanisms underlying it. In work recently published in the Journal of Cleaner Production, a team of researchers led by Dr Loredana Brinza at the Alexandru Ioan Cuza University of Iaşi in Romania investigated brown algae Fucus vesiculosus as a potential new bio sorbent for cadmium, Cd(II), removal from wastewater. Their results provide crucial information for process optimisation, pilot testing scaling up, and implementation of a clean, environmentally friendly biotechnology for wastewater treatments.
Dr Brinza and her colleagues are looking for new, more environmentally friendly bio sorbents for cleaning wastewater, reducing the need for chemically synthesised adsorbents that potentially create toxic waste themselves. This research investigates the potential use of algae as a bio sorbent to remove metals from wastewater contaminated through industrial processes such as mining and electroplating.
The team investigated cadmium uptake by Fucus vesiculosus, a very common kelp found abundantly on many seashores worldwide. This work focused on kelp collected from the Irish Sea, but previous studies tested the same species from other locations.
They performed quantitative and qualitative investigations to assess the kelp's maximum uptake capacity via adsorption under various environmental conditions and understand the uptake mechanism at the atomic scale, i.e. exactly how cadmium binds to the algal surface.
Understanding the binding mechanism of toxic metals at the algal surface after adsorption allows chemical and process engineers to manipulate the biosorbent in a very effective way. It enables them to select the best eluent to recover metals from the used biosorbent, recondition the biomass for reuse and safely dispose of it after its final use.
To map the spatial distribution and identify bonding environment of cadmium at the algal surface, the team used Diamond's I18 Microfocus Spectroscopy beamline.
Dr Brinza explains why:
The I18 beamline offers excellent spatial and analytical resolutions for most of the elements of the periodic table, together with appropriate environments for measuring biological samples. The X-Ray Fluorescence (XRF) measurements provide detailed mapping of metals of interest at high resolution, even for elements at low concentrations. The resulting XRF elemental maps allow correlation between elements of interest to be carried out and prove important guidance for further X-Ray Absorption Spectroscopy (XAS) measurements at the same station on the same sample.
XAS is another excellent technique, permitting identification of the bonding environment of elements of interest (i.e., valence, oxidation state, bonding distance between the main element of interest and neighbouring atoms, coordination numbers of the neighbouring atoms, etc.). It also allows us to determine the element speciation at the sample surface and within the sample chosen to be analysed. In this case, the XAS measurements tell us how cadmium binds at the algal surface and allow us to derive information about the binding mechanism at molecular scale.
The cryogenic sample environment on I18 allows researchers to investigate biological samples without using sample preparation techniques that may chemically alter them.
The results showed that, at the laboratory scale, dried Irish kelp has a superior uptake capacity to remove cadmium from wastewater, compared with other biosorbents and the chemically synthesised adsorbents currently in use. The team also demonstrated that the seaweed can be reused through several adsorption-desorption cycles. In previous studies, they found similar results for zinc uptake by Irish kelp from polluted wastewater.
Cadmium is bound by cellulosic fibres and alginate compounds (specifically by carboxyl and hydroxyl functional groups) from the algal cell wall. The contribution of these compounds to toxic metal uptake varies with wastewater pH, algae treatment between cycles and algae habitat.
Overall, their work shows that Fucus vesiculosus is an excellent bio sorbent to immobilise and remove Cd(II) from aqueous media, with applications to current water treatments, through future optimisation and scaling up to algal-based biotechnology.
To find out more about the I18 beamline or discuss potential applications, please contact Principal Beamline Scientist Konstantin Ignatyev: [email protected].
Brinza L. et al., The Irish kelp, Fucus vesiculosus, a highly potential green bio sorbent for Cd (II) removal: Mechanism, quantitative and qualitative approaches. Journal of Cleaner Production 129422 (2021). DOI:10.1016/j.jclepro.2021.129422.
Brinza L. et al., Zn adsorption onto Irish Fucus vesiculosus: Biosorbent uptake capacity and atomistic mechanism insights. Journal of Hazardous Materials, 365: p. 252-260 (2019). DOI: 10.1016/j.jhazmat.2018.11.009
Brinza L. et al., Baltic Fucus vesiculosus as potential bio-sorbent for Zn removal: Mechanism insight. Chemosphere 124652, 238, (2020). DOI: 10.1016/j.chemosphere.2019.124652
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Left: XRF elemental maps showing the microscale distribution of Cd(II) (red), K (green) and Ca (blue) onto the algae surface as collected from the sea (blank with no Cd, only K and Ca), (a) and after adsorption at: pH 5, CCd(II) 1 mM (b), pH 7, CCd(II) 1 mM (c), pH 9, CCd(II) 1 mM (d), pH 5, CCd(II) 10 mM (e), after the fourth cycle using 10 mM HCl, pH 5, CCd(II) 10 mM (f), after the third cycle using 10 mM NaOH, pH 5, CCd(II) 10 mM (g), and after the forth cycle using 1 mM EDTA, pH 5, CCd 10 mM (h). Marked on each map as white stars, from simple stars with 4 corners, stars with 5 corners to more complex ones with 6 corners, respectively, are the selected POIs, POI1, POI2 and POI3, at different Cd(II) concentrations, from which XANES spectra were collected. Step size resolution is 3 μm, and the map scale is in millimetres. Acquisition time per point is 1.5 s per point for map (a, e, f, g, h), and 0.5 s per point for maps (b, c, d).
Image copyright: © 2021 Copyright - All Rights Reserved, Elsevir:
https://www.sciencedirect.com/journal/journal-of-cleaner-production
Right: Cd(II) L3 adsorption edge XANES spectra of standards and the selected POIs marked on the XRF maps.
Image copyright: © 2021 Copyright - All Rights Reserved, Elsevir: https://www.sciencedirect.com/journal/journal-of-cleaner-production
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