One of the global challenges is the pollution of water bodies by heavy metals. Any metal and metalloid element having density within the range of 3.5 to 7 g cm− 3 is considered poisonous even if present in low concentrations (Gautam et al., 2014). An optimum low level of concentration of heavy metals such as iron (Fe), copper (Cu) and zinc (Zn) have biological usefulness while others, including lead (Pb) and cadmium (Cd), are not useful biologically and are toxic irrespective of the level of contamination (Halttunen, 2008).
Cadmium (Cd) is one of the most noxious heavy metals that could reach the food chain through absorption by plants from the soil (Pourret and Hursthouse, 2019; Penido et al., 2019). Human activities (e.g., mining, electronics and metallurgical industries) are sources of Cd that causes contamination of water and soil (Lam et al., 2019; Karunanayake et al., 2018; Guo et al., 2019). Nordberg et al. (2018) stated Cd absorbed from the soil by field crops, such as wheat, rice and potatoes, has negative effects on human bones. In Japan, water and soil contaminated by Cd is the major cause of the Itai-itai disease (Khan et al., 2020). Cadmium causes bone disease, kidney damage and cancer. It is reported that high levels of Cd exposure causes osteoporosis, renal dysfunction and liver damage (Ghoneim et al., 2014).
Alkaline batteries manufacturing consumes about three-quarters of Cd production. The remaining one quarter of Cd production is used by processes including coating materials and as a plastic stabilizer (Jaishankar et al., 2014). Cadmium is extremely poisonous, causes plant nutrient deficiency and oxidative stress, and also impacts on the enzymatic systems of cells (Jaishankar et al., 2014; Irfan et al., 2013). Therefore, WHO (2008) recommends that the concentration of Cd in potable water should be limited to 3 µg L− 1 (Zinicovscaia, 2016). For the short- and long-term irrigation water use, the preferred threshold of Cd concentration should be 0.05 and 0.01 mg L− 1, respectively (Fipps, 2015; Rowe and Abdel-Magid, 1995).
Human health issues related to the pollution of water bodies and soil by heavy metals resulting from pesticides, fertilizers, sewage water and industrial activities have received global attention. There is lack of precautionary measures put in place to inspect industrial facilities that discharge contaminated wastewater into agricultural drains that supply irrigation water for production of crops in many countries, be it developing or developed. Thus, people who handle the contaminated irrigation water and the resulting agricultural products put their health at risk (Tchounwou et al., 2012; Dadar et al., 2016; Avigliano et al., 2015). Many different methods of wastewater treatment (physical, chemical treatment, biological and phytoremediation) have been applied to reduce cadmium concentration in water to the recommended international standards (Chen et al., 2020). Some of the wastewater treatments applied include physicochemical processes (ion exchange and chemical precipitation), electrochemical treatments (electrocoagulation, electrodeposition and elector-floatation), adsorption (carbon nanotubes, activated carbon and wood sawdust adsorbents), and the current methods are photocatalysis, membrane filtration, and nanotechnology (Azimi et al., 2017).
However, these techniques have many disadvantages. For example, they are costly, consume high energy, have potential of secondary pollution and are only valid within a range of Cd concentration (Lamet al., 2019; Khan et al., 2020). Moreover, most of the conventional techniques do not provide 100% heavy metal removal with restrictions on pH variation (Ahluwalia and Goyal, 2007; Kumar et al., 2015), are efficient at only smaller concentrations (Kumar et al., 2015; Antunes et al., 2003), produce toxic sludge and waste products, and need high exploitation and recycling costs (Oboh et al., 2009; Kumar et al., 2015). Being environmentally friendly, biotechnology techniques have received significant attention. In addition, they provide high heavy metal removal efficiency, consume less energy, and are carried out at lower pressure and temperature (Fulekar, 2010; Singh et al., 2014). However, availability of selected biomass types for the biosorbent is of primary importance. Biosorbents can be nature based (e.g., bacteria, fungi and algae) or from industrial and agricultural wastes. Several studies have used biosorbents for dye and metal treatments (Vieira, 2000; Kumar and Pavithra, 2018).
Phytoremediation is any process that use algae to bioremediate contaminated water and wastewater (Phang et al., 2015). The characteristics of algae involve a high ratio of surface area to volume, high heavy metals tolerance, growth possibility either autotrophically or heterotrophically, the ability for genetic manipulation, phytochelatin expression and phototaxy (Kumar and Gunasundari, 2018). Biosorption using blue-green algae (cyanobacteria) is rich in vitamins and proteins. The biomass can absorb and adsorb heavy metals from aquatic solution even when the cells are dead. Unlike conventional methods, cyanobacteria processes do not produce polluting sludges, are highly effective, easy to operate and cost effective for treating large quantities of wastewater with low contaminant concentrations (Garnham et al., 1992; Prakash and Awasthi, 2013). Cyanobacteria are exemplary biosorbents and are commonly found in ecosystems of water and soil (Chojnacka and Noworyta, 2004; Cepoi et al., 2016).
There were three objectives of this study; i) evaluation of the removal efficiency of Cd in aqueous solution by two cyanobacteria species (Nostoc muscorum and Anabaena variabilis) in a specially designed laboratory pilot scale experiment, ii) investigation of the influence of various concentrations of Cd on algae growth parameters, and iii) treatment of wastewater to the world health organization (WHO) standard for reuse for agrarian activities.