Phytoremediation of Cr(VI) and Pd(II) using vetiver grass (Chrysopogon Zizanioides)

Abstract

Heavy metal contamination from industrial effluents is one of the leading causes of concern. Chromium(VI) is amongst the top chemical pollutants that are released into the environment from serval industries. Chromium(VI) has devastating health effects on living organisms. Despite their scarcity, platinum group metals (PGMs) are also finding their way into the environment. Their health effects and short supply are the main causes of concern given the continuous growth in demand and continuous disposal into the environment. There is a need for environmentally friendly technologies that can remediate these metals from wastewaters and the environment without breaking the bank. Phytoremediation is an emerging cost-effective and environmentally friendly technology compared to conventional remediation techniques. Phytoremediation make use of plants to take up metals from the environment. The exact mechanisms by which plants take up and distribute toxic metals is not well understood. Several factors such as metal concentration, physicochemical properties of the environment, and the presence of other ions in the roots zone, affect the bioavailability of metals. It has been reported that carriers responsible for the uptake of essential ions are also involved in the uptake of non-essential metals. Plants restrict the translocation of toxic metals to the aerial parts to combat their detrimental effects.

The present study aimed to assess the phytoremediation potential of Chrysopogon zizanioides (vetiver grass) for chromium(VI) and palladium(II) from water. The study was conducted in four phases of batch experiments. The first phase was to access the effect Cr(VI) initial concentration on the growth of vetiver grass (VG), uptake of Cr(VI), and accumulation of chromium in the plant. Using different Cr(VI) concentrations from 5 ppm to 70 ppm. From this study it was found that the initial concentration had an effect on the uptake and translocation of chromium in vetiver grass. The grass exposed to 70 ppm managed to accumulate 1.12 mg g1 in the roots and 3.04 mg g1 in the leaves. The toxicity of chromium was visible at concentrations above 30 ppm, resulting in withering of the grass.

The second phase investigated the effect of plant density on the uptake of chromium at 30 ppm initial concentration. The densities were grouped in the following classes, low density (5 slips), medium density (10 slips), high density (15 slips). The uptake of Cr(VI) increased with an increase in plant density, with the high density pot achieving a removal of 26.1 ppm Cr(VI) from initial concentration of 31.9 ppm. The accumulation of chromium in the roots increased with the plant density. While the accumulation in the leaves decreased with an increase in plant density.

The third phase focused on the effect of pH on the uptake and accumulation of chromium. The pH levels ranged from 3.5 to 10.5 at initial Cr(VI) concentration of 30 ppm. The grass exposed to pH of 3.5 achieved 100 % Cr(VI) removal. The accumulation of Cr decreased in the roots and increased in the leaves as the solution pH increased. The translocation to the leaves was minimal under different pH levels.

The last phase was to evaluate the phytoextraction of palladium(II) a precious metal at different initial concentrations. The concentrations ranged from 10 ppm to 120 ppm. The uptake of Pd(II) increased with the initial concentration, the grass exposed to 120 ppm managed to reduce the pot concentration by 24 ppm, while accumulating the most palladium of 0.4 mg g1 in the roots and 0.15 mg g1 in the leaves. The toxicity of palladium was mainly visible at concentrations above 40 ppm resulting in the drying up of the exposed grass.

The results from the current study proved the potential of VG in phytoremediation of Cr(VI) and Pd(II). Phytoextraction potential of chromium proved possible at elevated concentrations and extended periods as demonstrated in the first experiments, while more research is required to access vetiver grass’s potential in phytoextraction of palladium. The ability of VG to accumulate metal in its aerial parts showed the potential of applying VG in extracting and recovering metals for reuse by burning and reprocessing of ash. The latter is common practice in other industries such as the mining and refining of platinum and other platinum group metals (PGMs).