Recovery of Au from dilute aqua regia solutions via adsorption on the lyophilized cells of a unicellular red alga Galdieria sulphuraria: A mechanism study

Significance and Applications of the Thermo-Acidophilic Microalga Galdieria sulphuraria (Cyanidiophytina, Rhodophyta)

Galdieria sulphuraria is a thermo-acidophilic microalga belonging to the Cyanidiophyceae (Rhodophyta) class. It thrives in extreme environments, such as geothermal sulphuric springs, with low pH, high temperatures, and high salinity. This microalga utilises various growth modes, including autotrophic, heterotrophic, and mixotrophic, enabling it to exploit diverse organic carbon sources. Remarkably, G. sulphuraria survives and produces a range of bioactive compounds in these harsh conditions. Moreover, it plays a significant role in environmental remediation by removing nutrients, pathogens, and heavy metals from various wastewater sources. It can also recover rare earth elements from mining wastewater and electronic waste. This review article explores the diverse applications and significant contributions of G. sulphuraria.

Mining wastewater treatment technologies and resource recovery techniques: A review

Mining wastewater can have adverse effects on the ecosystem; thus, treatment before discharging into the environment is of utmost importance. This manuscript reports on the effect of mining wastewater on the environment. Moreover, the currently used, effective and commercialised mine wastewater treatment technologies such as SAVMIN®, SPARRO®, Biogenic sulphide, and DESALX® are reported in this study. These technologies integrate two or more separation processes, which have been proven to be effective for the high recovery of salts and water for reuse. Some of the technologies reported can significantly recover salts and >95% of water. Modern pilot-stage and laboratory-scale treatment systems used for the recovery and removal of metals are also reported herein. Since some treatment technologies can generate highly toxic sludge and other waste products, the management of the generated waste was also considered. Some studies have focused on the treatment of wastewater at the laboratory level using the adsorption process. Most adsorbents exhibit promising results; however, there is insufficient research on reusability, toxic sludge management, and the economic analysis of the systems. Moreover, the implementation of adsorption systems in wastewater is necessary. Furthermore, the integration of treatment systems to recover precious metals at low concentrations is desirable in addition to water reclamation to achieve circular mine water.

Reversible adsorption of iridium in lyophilized cells of the unicellular red alga Galdieria sulphuraria

Iridium (Ir) is one of the rarest elements in the Earth's crust and is valuable in industry due to its high corrosion resistance. In this study, we used lyophilized cells of a unicellular red alga, Galdieria sulphuraria for the selective recovery of small amounts of Ir from hydrochloric acid (HCl) solutions. The Ir recovery efficiency of the lyophilized cells was higher than that of activated carbon and comparable to that of an ion-exchange resin in up to 0.2 M acid. Lyophilized G. sulphuraria cells showed different selectivity from the ion-exchange resin, adsorbing Ir and Fe in 0.2 M HCl solution while the ion-exchange resin adsorbed Ir and Cd. The adsorbed Ir could be eluted with more than 90% efficiency using HCl, ethylenediaminetetraacetic acid, and potassium hydroxide solutions, but could not be eluted using a thiourea-HCl solution. After the elution of Ir with a 6 M HCl solution, lyophilized cells could be reused up to five times for Ir recovery with over 60% efficiency. Scanning electron-assisted dielectric microscopy and scanning electron microscopy revealed that Ir accumulated in the cytosol of the lyophilized cells. X-ray absorption fine structure analysis demonstrated the formation of an outer-sphere complex between Ir and the cellular residues, suggesting the adsorption via ion exchange, and explaining the ability to elute the Ir and reuse the cells. Our results provide a scientific basis for inexpensive and environmentally friendly biosorbents as an alternative to ion-exchange resins for the recovery of Ir.

Galdieria sulphuraria ACUF427 Freeze-Dried Biomass as Novel Biosorbent for Rare Earth Elements

Rare earth elements (REEs) are essential components of modern technologies and are often challenging to acquire from natural resources. The demand for REEs is so high that there is a clear need to develop efficient and environmentally-friendly recycling methods. In the present study, freeze-dried cells of the extremophile Galdieria sulphuraria were employed to recover yttrium, cerium, europium, and terbium from quaternary-metal aqueous solutions. The biosorption capacity of G. sulphuraria freeze-dried algal biomass was tested at different pHs, contact times, and biosorbent dosages. All rare earths were biosorbed in a more efficient way by the lowest dose of biosorbent, at pH 4.5, within 30 min; the highest removal rate of cerium was recorded at acidic pH (2.5) and after a longer contact time, i.e., 360 min. This study confirms the potential of freeze-dried cells of G. sulphuraria as innovative ecological biosorbents in technological applications for sustainable recycling of metals from e-waste and wastewater.

Cell population behavior of the unicellular red alga Galdieria sulphuraria during precious metal biosorption

This study investigates the biosorption mechanism, including cell population behavior, of trace amounts of precious metals (gold, palladium, and platinum) in a unicellular red alga, Galdieria sulphuraria. Single-cell inductively coupled plasma mass spectrometry showed that the number of adsorbing cells and the concentration of adsorbed metal per cell varied depending on solution acidity and metal species. The X-ray absorption fine structure in 5 mM HCl solution indicated that the adsorbed Au formed inner-sphere complexes with S, whereas the adsorbed Pd and Pt formed an inner-sphere complexes with N and/or S. In 500 mM HCl solution, the adsorbed Au and Pd formed inner-sphere complexes only with S, and the Au formed a structure similar to Au₂S. At higher acidity, Au and Pd were recovered by interacting with residues that formed more stable complexes, which was accompanied by changes in the behavior of cell populations adsorbing the metals. This is the first study to demonstrate the relationship between changes in the behavior of cell populations and chemical interactions that occur between substrate elements and biomaterial residues during biosorption. The findings of this study provide deeper insights into the biosorption mechanism and a background for the design of an environmentally friendly biosorbent.

Extraction of Au(iii) from hydrochloric acid media using a novel amide-based ionic liquid

A study on the performance of selective extraction of Au(iii) using a novel amide-based IL.