With growing urbanization and ongoing development activities, the consumption of heavy metals has been increasing globally. Although heavy metals are vital for the survival of living beings, they can become hazardous when they surpass the permissible limit. The effect of heavy metals varies from normal to acute depending on the individual, so it is necessary to treat the heavy metals before releasing them into the environment. Various conventional treatment technologies have been used based on physical, chemical, and biological methods. However, due to technical and economic constraints and poor sustainability towards the environment, the use of these technologies has been limited. Microalgal-based heavy metal removal has been explored for the past few decades and has been seen as an effective, environment-friendly, and inexpensive method compared to conventional treatment technology. Cyanidiales that belong to red algae have the potential for remediation of heavy metals as they can withstand and tolerate extreme stresses of heat, acid salts, and heavy metals. Cyanidiales are the only photosynthetic organisms that can survive and thrive in acidic mine drainage, where heavy metal contamination is often prevalent. This review focuses on the algal species belonging to three genera of Cyanidiales: Cyanidioschyzon, Cyanidium, and Galdieria. Papers published after 2015 were considered in order to examine these species’ efficiency in heavy metal removal. The result is summarized as maximum removal efficiency at the optimum experimental conditions and based on the parameters affecting the metal ion removal efficiency. This study finds that pH, initial metal concentration, initial algal biomass concentration, algal strains, and growth temperature are the major parameters that affect the heavy metal removal efficiency of Cyanidiales.

The strain of red microalgae Galdieria sulphuraria CCMEE 5587.1 was evaluated in a controlled laboratory environment for its ability to tolerate and remove two heavy metal (HM) ions: cadmium [Cd(II)] and lead [Pb(II)] in aqueous solutions as a single metal species. Various concentrations (0 mg L⁻¹ to 5 mg L⁻¹) of Cd and Pb ions were added to the Cyanidium medium in which the chosen microalgae strain G. sulphuraria CCMEE 5587.1 was grown at an acidic pH of 2.5. The effectiveness of G. sulphuraria CCMEE 5587.1 in tolerating and removing these two metal ions was measured by analyzing its growth profile, growth rate, nutrient removal, and metal ion removal efficiency. The growth of G. sulphuraria CCMEE 5587.1 was inhibited during the initial days of incubation, and the growth rate decreased when the HM concentration in the media was increased. Nutrient removal in the HM-containing media is comparable to that in the control media at low metal concentrations but decreases as the metal concentration rises. G. sulphuraria CCMEE 5587.1 has the highest removal efficiency for Cd and Pb in a medium containing 2.5 mg L⁻¹ of metal ions, which is 49.80% and 25.10%, and the corresponding sorption capacity is 1.45 mg g⁻¹ and 0.53 mg g⁻¹ of dry biomass, respectively. These findings suggest that G. sulphuraria CCMEE 5587.1 holds potential as a viable bioremediation solution for extracting Cd and Pb from wastewater, alongside its capacity to remove nutrients concurrently. The study underscores the dual advantage of G. sulphuraria CCMEE 5587.1, making it a promising candidate for addressing heavy metal pollution in wastewater treatment processes.

This study examined potential of using wheat straw ash (WSA) and rice husk ash (RHA) to remove hardness of natural and hardness enriched synthetic water samples. Eight water samples (both natural and synthetic) with hardness ranging from 236 to 580 mg/l were treated with 10 different doses of WSA and RHA ranging from 2.5 to 25 g ash l–1 at regular increment of 2.5 g and one sample was included as control (no ash addition). Water hardness, hardness removal efficiency and pH changes of the treated water samples were measured. Hardness removal efficiency and pH of water increased with increasing doses of ash. Maximum removal was achieved at 17.5 g/l and 22.5 g/l for WSA and RHA, respectively, and further ash addition did not change the hardness. Maximum removal efficiency of WSA was 81% (i.e. 67 mg/g) and RHA was 58% (i.e. 44 mg/g). Higher hardness removal efficiency of WSA was due to presence of higher concentrations of alkali metal oxides. As similar to removal efficiency, WSA also increased pH of water samples relative to RHA. Maximum pH of 11.2 and 10.2 was respectively for WHA and RSA at the highest dose of ash (25 g/l). The results thus indicate that locally available ashes from agricultural sources may be useful in hardness removal process, however, softened cannot be used as drinking water due to the high level of alkalinity.

This study aims to evaluate the potential of using a thermophilic acidophilic red alga, Galdieria sulphuraria for effective on-site treatment of municipal landfill leachate (LL). This study focused on evaluating the effects of LL dilution, nitrogen loading, and initial algal biomass density on the overall treatment efficiency, and evaluated the long-term performance of the system using 5-day growth cycles. This study confirmed that optimal conditions for G. sulphuraria biomass production are 20% strength LL, a lower initial biomass concentration of 0.25 g L⁻¹, and the addition of N at twice the level of initial media. Furthermore, the results indicated G. sulphuraria’s ability to grow in elevated NH₄-N concentration (>950 mg L⁻¹) and provide nitrogen removal rates of up to 40 mg L⁻¹ d⁻¹. In addition, the long-term running experiment showed that the proposed algal-based system could be applied in semi-continuous mode to achieve bioremediation. Overall, the results obtained from this study can be used to develop the necessary process parameters to implement large-scale algal-based systems for landfill leachate treatment.

The release of cadmium into the environment is a significant global concern due to its toxicity, non-biodegradability, and persistence in nature. There is an urgent need for effective, eco-friendly, and cost-effective systems for removing Cd because of the many drawbacks of conventional physicochemical techniques. This study investigated the ability of the extremophile red microalgal strain Galdieria sulphuraria CCMEE 5587.1 to tolerate and remove Cd (II) ions at acidic pH in a controlled laboratory environment. Three distinct concentrations of Cd (1.5 mg L⁻¹, 3 mg L⁻¹, and 6 mg L⁻¹) were introduced to the cyanidium medium, and G. sulphuraria cells were introduced in the medium and grown for ten days. Four distinct aspects were identified regarding Cd removal: time course Cd removal, total Cd removal, extracellular Cd removal, and intracellular Cd removal. The inhibitory effects of Cd on G. sulphuraria growth were observed using a daily growth profile. Initial incubation days showed an inhibition of G. sulphuraria growth. In addition, increasing the Cd concentration in the medium decreased the growth rate of G. sulphuraria. Rapid Cd removal occurred on the first day of the experiment, followed by a steady removal of Cd until the last day. The highest total removal efficiency occurred in a medium containing 3 mg L⁻¹ of Cd ions, which was 30%. In contrast, the highest sorption capacity occurred in a medium containing 6 mg L⁻¹ of Cd ions, which was 1.59 mg g⁻¹ of dry biomass. In all media compositions, a major fraction (>80%) of Cd removal occurred via adsorption on the cell surface (extracellular). These results showed that G. sulphuraria cells can remove Cd ions from aqueous solution, which makes them a potential bioremediation option for heavy metal removal.

A rural catchment stream, Roshi (Nepal), is experiencing pollution loads from non-point pollution sources: agricultural runoffs and stone quarry sediment loads requiring immediate concern. This study is focused on developing a well-calibrated and validated water quality model to determine the pollution load capacity of Roshi River at peak rainfall periods using software QUAL2kW concerning pH, Temperature, DO, FSS, NO3-, NH4+, TP, BOD, and Turbidity. A pollution-dominant stretch of 1.95km was selected and segmented into three segments. RMSE values showed the model satisfactory in predicting actual in-field scenarios. Scenario analysis showed that in the existing pollution source flow and streamflow, the pollution load capacity is 275 mg/L for BOD, 33 mg/L for NH4+, 1250 mg/L for NO3-, 10 mg/L for TP, and 0 mg/L for DO. The FSS in the Headwater must be limited to 55 mg/L to maintain the stream water quality within standards signifying the immediate need to reduce the upstream stone quarry load. Policymakers are urged to implement stringent regulations on upstream quarries concerning sediment load discharge into the stream. The developed model can be a valuable tool for predicting the stream's response to potential emissions from future development activities during the monsoon period.