Microalgae-based green approach for effective chromium removal from tannery effluent: A review

Tannery effluent can cause serious environmental pollution due to its high levels of chromium, chemical oxygen demand (COD), biochemical oxygen demand (BOD) and other pollutants like nitrogen, phosphorus, dyes, sulphur etc. The problem of chromium in tannery effluent is that it is highly toxic and can be hazardous to aquatic life, wildlife, and human health. High levels of chromium can disrupt the ecological balance of a water body, leading to problems such as reduced aquatic life diversity. Additionally, chromium can enter the food chain from water sources and can potentially have negative effects on humans. Therefore, comprehensive waste management plans are highly required to reduce the environmental impact of tannery effluent. The conventional physicochemical methods for removing Cr from wastewater may generate a vast amount of sludge leading to sludge disposal issues. Besides, these techniques require huge capital investments, and the applied chemicals may cause secondary pollution. On the other hand, the biological removal of Cr from wastewater is sustainable, environmentally friendly, and cost-effective. Biological means of Cr remediation involve plant-based and microorganism-based approaches, including bacteria, fungi, and microalgae. This review emphasizes the adverse effects of chromium on human health and its remediation processes based on microalgae. Microalgal biomass can remove heavy metals, including Cr, in either a living (bioaccumulation) or non-living (biosorption) state. There are a number of microalgae species that can remove chromium from wastewater. Among them several algae species, including Chlorella vulgaris, Scenedesmus sp., Consortium of Phormidium sp. and Chlorella sp., Chlorella sorokiniana, and Arthrospira platensis (Spirulina), have found successful in removing Cr in the range of 100–73.5% from tannery wastewater. Additionally, it can mitigate CO2, can remove phosphorus, nitrogen, COD and BOD from tannery effluent, improving its quality to be discharged into the environment. Thereby, the use of microalgae is thought to be a viable substitute for traditional techniques to mitigate the problem of environmental pollution caused by tannery effluent. However, treatment efficiency can be depending on several factors such as nature and amount of effluent to be treated, environmental conditions, contact time, and amount of optimal algal biomass