The Potential of Pistia stratiotes in the Phytoremediation of Selected Heavy Metals from Simulated Wastewater

Bioconcentration factor; Phytoremediation; Pistia stratiotes; Relative treatment efficiency index; Translocation factor

Heavy metals are hazardous, as they can affect human health, plant growth and biodiversity. Heavy metals can be classified into non-essential and essential metals. Heavy metals such as cadmium (Cd) and lead (Pb) are considered non-essential heavy metals that are not necessary for humans and animals (Tamele and Loureiro, 2020). Zinc (Zn) and nickel (Ni) are essential heavy metals for human metabolism; nevertheless, they can cause unfavorable effects when they exceed certain threshold levels (Edelstein and Ben-Hur, 2018). Water pollution by heavy metals is non-biodegradable, as they can exist in diverse oxidation states that can last long in the environment (Yadav et al., 2018).

        As Malaysia is a fast-growing country with rapid industrialization and urbanization, this encourages an increase in the generation of heavy metals (Firmawan et al., 2012). The improper disposal of these contaminants can be transported to water bodies through the runoff of untreated metals introduced into open water bodies, which eventually causes water pollution (Suwartha and Pujiastuti, 2017; Li et al., 2019). In 2015, Malaysia spent RM 2.6 billion to protect these sectors from polluting the environment: 73.6% for manufacturing industries, 11.3% for public services, 6.9% for mining and quarrying, 6.4% for construction and another 1.8% for agriculture, forestry and fishing (Mahidin, 2018). From these statistics, the manufacturing industry is the sector of most concern for environmental pollution, and specifically water pollution. There are some treatment technologies that have been established to isolate heavy metals from water, such as membrane filtration, coagulation-flocculation, precipitation, ion exchange and adsorption (Hudaya et al., 2018). These conventional or physico-chemical techniques for treating heavy metals are time-saving, but they require a high operational cost as well as extra costs for sludge disposal (Barakat, 2011). Phytoremediation is an eco-friendly and cost-effective technique for utilizing plants in removing heavy metals from polluted water bodies without generating sludge during the treatment process (Zhang et al., 2018). In this study, Pistia stratiotes, a type of free-floating aquatic macrophyte that belongs to the Araceae family, was chosen based on its special characteristics, such as its high rate of growth, high tolerance to heavy metals and ease of cultivation. This plant can easily be found in tropical and sub-tropical areas, where the abundance of sunlight and warm temperatures encourage its growth to the point where it can be considered an invasive species and a nuisance (GISD, 2005).  A laboratory experiment was conducted in a 40L reactor tank cultivated with P. stratiotes for 14 days to treat selected heavy metals taken from simulated wastewater. The accumulation of heavy metals in P. stratiotes tissues was examined by the end of the treatment. The present study also highlighted the relative treatment efficiency index (RTEI) and the feasibility for Pistia stratiotes to thrive in high concentrations of heavy metals up to 3 mg/L. This work can provide information on the defense mechanism of the plant in coping with heavy metals, namely Cr, Pb and Ni.  This data can be used in pilot aquatic plant-based wastewater treatment plants. As Pistia stratiotes can easily be found in Sabah Malaysia, the use of this plant for remediation is an appealing green technology for treating the accumulation of different heavy metals.

    The obtained results show that P. stratiotes has the highest tolerance and removal efficiency for lead, with a removal percentage of 99.31% at 1 mg/L. This plant can accumulate chromium and nickel, although it may not survive at higher concentrations of heavy metals nor achieve a removal efficiency greater than that of lead. The RTEI values proved that P. stratiotes is capable of removing all three heavy metals tested in this study. For BCF and TF values, it was found that the phytoremediation mechanism uptake of P. stratiotes is rhizofiltration. The ability of P. stratiotes to remove heavy metals especially lead had indicated their potential in treatment of metal polluted water. It is important to note that the plants must be harvested regularly to avoid it releasing heavy metals back into the environment.

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