The effects of heavy metals on the quality and yield of vetiver grass (chrysopogon zizanioides) essential oil

Abstract

Vetiver grass (Chrysopogon zizanioides;CZ) is a well-known grass that can both remediate various heavy metals in soil as well as producing essential oil that is commonly used in premier perfumery, food and pharmaceutical industries. Due to this versatility property, this study was aimed at extracting essential oil from Chrysopogon zizanioides roots cultivated in SelibePhikwe mine tailings and study the quality and quantity (chemical composition and yield) of the Chrysopogon zizanioides essential oil in relation to toxic multi heavy metals absorbed by the grass assisted by Arbuscular Mycorrhizal Fungi (AMF) and ethelene Diamine Tetra-Acetic Acid (EDTA) chelating agent. The experimental set-up was designed in a complete randomized design with triplicates. The ultrasonic -assisted hydro distillation (UAHD) extracted CZ essential oil was analysed in each treatment using Gas Chromatography Flame Ionization Detector (GC-FID). Cumulatively fifty-nine constituents were identified from the seven treatment samples, however few constituents which mostly were in trace amount were not identified since they were not reported in similar studies. The essential oil was found to contain mainly sesquiterpenes class of compounds. Control, T1, T2, T3, T4, T5, T6 shows the total identified oils as 99.5%, 97.0%, 100%, 94.6%, 100%, 98.1% and 97.2% respectively. The predominating constituents are alcohols (>50%) across all the treatments. The principal constituents vary in all the treatments but the ‘finger print’ constituents; khusimol, ά-vetivone, β-vetivone appears in > 50% of the treatment samples. The percentage yield ranged between 0.26% to 0.95%. Two factor ANOVA in complete Randomised Design showed that statistically, the percentage yields across all the treatments are very highly significant (P =0.000, P < 0.05). Microwave acid assisted digestor (MAAD) was used to extract heavy metals from raw CZ roots and extracted essential oil and The heavy metal analysis by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) showed that across all the six treatments with CZ grass grown in mine tailing containing multi heavy metals, the accumulation level is as follows: Ni>Cu>Mn>Zn>Pb>As for raw CZ roots but the cumulative essential oil contained low to negligible amount of heavy metals, as the concentration were; Cu (0.150 ± 0.007 mg kg-1 ), Ni (0.063 ± 0.001 mg kg-1 ), Pb (0.022 ± 0.002 mg kg-1 ), As (0.003 ± 0.003 mg kg-1 ), Zn (0.109 ± 0.002 mg kg-1 ), Mn (0.043 ± 0.001 mg kg-1 ). It shows that the heavy metals were not co extracted with essential oil. There has been a significant increase in the number of constituents produced by CZ grass for the treatments with CM only and inoculation of AMF with 5 mmol EDTA single and a significant decrease for treatments with AMF and 5 mmol EDTA split, EDTA single, 5 mmol EDTA split as well as AMF only as a result of high metal stress and low metal respectively. The results showed that the amount of heavy metal content does not all the time and it’s not the only factor influencing the production of chemical constituents. The integrated effect of Ni, Cu, Mn, Zn, Pb and As in overall did not significantly alter the essential oil composition of most of the CZ constituents across most of the treatments as compared to the control treatment since precision was high. The extracted light brown viscous CZ essential oil showed percentage yield of 0.26% (C), 0.36% (T3), 0.54% (T5), 0.86% (T1), 0.94% (T6) and 0.95% (T2). The trend shows that T1, T2, T4, T6 had precise and better yield while for C, T3 and T5 the yields dropped with C being the least. The treatments with no heavy metals nor AMF + EDTA (C), the yield was low due to no or less metal stress triggering the production of essential oils.