Improving capacity for phytoremediation of Vetiver grass and Indian mustard in heavy metal (Al and Mn) contaminated water through the application of clay minerals

Abstract

One of the consequences of mining is the release of heavy metals into the environment, especially water bodies. Phytoremediation of areas contaminated by heavy metals using Vetiver grass and Indian mustard is cost-effective and environmentally friendly. This study aimed at enhancing remediation of heavy metal contaminated water through the simultaneous hybrid application of clay minerals (attapulgite and bentonite) and Vetiver grass or Indian mustard. A 21-day greenhouse experiment was carried out to investigate the effectiveness of the clay minerals to improve heavy metal phytoremediation. The highest accumulation of aluminium (Al) by Vetiver grass was 371.8 mg/kg in the BT2.5VT treatment, while for Mn, the highest accumulation of 34.71 mg/kg was observed in the AT1VT treatment. However, Indian mustard showed no significant uptake of heavy metals, but suffered heavy metal toxicity despite the addition of clay minerals. From this study, it was evident that bentonite added at 2.5% (w/v) could improve the phytoremediation capacity of Vetiver grass for Al and Mn polluted water. The current laboratory-scale findings provided a basis for field trials earmarked for remediation in a post-mining coal environment in South Africa. This remediation approach can also be adopted in other places.

Keywords: Contaminated water; Heavy metals; Indian mustard; Phytoremediation; Vetiver grass.

Fig. 1

Tolerance index (TI) of (a) Vetiver grass and (b) Indian mustard in the various treatments at the end of the experiment. Data shown as mean ± SD of triplicates (Error bars represent standard deviation). Lower case letters on top of error bars indicate statistically significant variance between means. Key: VT- Vetiver grass only; AT1VT- Vetiver + attapulgite applied at 1% (w/w); AT2.5VT- Vetiver + attapulgite applied at 2.5% (w/w); BT1VT- Vetiver + bentonite applied at 1% (w/w); BT2.5VT- Vetiver + bentonite applied at 2.5% (w/w); BJ- Indian mustard only; AT1BJ- Indian mustard + attapulgite applied at 1% (w/w); AT2.5BJ- Indian mustard + attapulgite applied at 2.5% (w/w); BT1BJ- Indian mustard + bentonite applied at 1% (w/w); BT2.5BJ- Indian mustard + bentonite applied at 2.5% (w/w)

Fig. 2

Concentrations of manganese (Mn) in (a) roots and (b) shoots of Vetiver grass in the different treatments at the end of the experiment. Data shown as mean ± SD of triplicates (Error bars represent standard deviation). Lower case letters on top of error bars indicate statistically significant variance between means (ρ < 0.05) based on Tukey’s Honest Significant Difference (HSD) test. Data with non-significant variance have the same letter. Key: VT: Vetiver grass only; AT1VT: Vetiver + attapulgite (1% w/v); AT2.5VT: Vetiver + attapulgite (2.5% w/v); BT1VT: Vetiver + bentonite (1% w/v); BT2.5VT: Vetiver + bentonite (2.5% w/v)

Fig. 3

Concentrations of aluminum (Al) in (a) roots and (b) shoots of Vetiver grass in the different treatments at the end of the experiment. Data shown as mean ± SD of triplicates (Error bars represent standard deviation). Lower case letters on top of error bars indicate statistically significant variance between means (ρ < 0.05) based on Tukey’s Honest Significant Difference (HSD) test. Data with non-significant variance have the same letter. Key: VT: Vetiver grass only; AT1VT: Vetiver + attapulgite (1% w/v); AT2.5VT: Vetiver + attapulgite (2.5% w/v); BT1VT: Vetiver + bentonite (1% w/v); BT2.5VT: Vetiver + bentonite (2.5% w/v)

Fig. 4

Principal component analysis (PCA) for the accumulation of Al and Mn in the different treatments. Key: VT: Vetiver grass only; AT1VT: Vetiver + attapulgite (1% w/v); AT2.5VT: Vetiver + attapulgite (2.5% w/v); BT1VT: Vetiver + bentonite (1% w/v); BT2.5VT: Vetiver + bentonite (2.5% w/v)