Evaluation of vetiver phytoremediation ability to absorb heavy metals As, Hg and Cr in industrial wastewater

The water sources surrounding the industrial areas are easily contaminated by industries wastewater in Iran as around the world. Almost all wastewaters in industrial areas are discharged untreated directly into rivers, lakes and neighbor lands. Industrial wastes contain very high levels of both organic and inorganic compounds. Because of the complexity, presence of different kinds of toxic heavy metals and difficulties of refining process for industrial wastewaters, they have more challenges compared with urban wastewater. Depending to the type of industrialization, industrial wastewaters contain various heavy metals and organic chemicals that potentially can affect the human and ecosystem health by direct and indirect pathways. Recently, the Vetiver grass has been successfully introduced as an environmental protection, particularly in the field of wastewater treatment, due to its morphological and physiological properties and also its high tolerance to the adverse edaphic conditions. Its effectiveness, simplicity and cost-effective application made the Vetiver grass technology as a good alternative treatment for phytoremediation and rehabilitation of the domestic, municipal and industrial wastewater. This system of remediation has not deeply studied for industrial wastewaters in Iran. Therefore, the main objective of this study was to use the vetiver grass as a simple and an inexpensive method of phytoremediation to absorb and remove the heavy metals, comprising the As, Hg and Cr, from industrials wastewater.
The wastewater for this study was collected from wastewater outlet of Borazjan Industrial park. All pots were filled with the clay loam soils of Dalki alluvial plains and also 5 % (W/W) of manure was incorporated in each pot. The vetiver grass was grown outdoors and irrigated with usual irrigation water for 30 days. After that, irrigation water treatments have been done to a completely randomized design with 8 treatments in 4 replications for three times per weeks up to 75 days. The treatments were: 100 % usual irrigation water (A); 100 % wastewater (B); 50% usual irrigation water% wastewater (C); 70% usual irrigation water% wastewater (D); 30% usual irrigation water% wastewater (E); interval irrigation with usual irrigation water and wastewater (F); two times irrigation with wastewater and one time with usual water (G); and two times irrigation with usual water and one time with wastewater (H). Later on, concentration of Arsenic (As), Mercury (Hg) and Chromium (Cr) were measured in soils and also determined in the roots and shoots of vetiver grass by atomic absorption spectroscopy (AAS).
Results showed that the highest and lowest concentrations of heavy metals were observed in B and A treatments, respectively. The highest concentration of As, Hg and Cr in soils was 401.75, 0.019 and 0.6 (mg kg-1), respectively. The lowest observed concentration of As, Hg and Cr were 3.5, 0.001 and 0.017 (mg kg-1), respectively. Moreover, the highest and lowest concentration of As, Hg and Cr in roots of vetiver grasses were 85.68 and 0.62 (mg kg-1), 0.01 and 0.001 (mg kg-1) and 7.09 and 0.0005 (mg kg-1), respectively. The accumulation of Hg in roots and shoots has not any significant differences between treatments. The Hg contents in roots are significantly higher in plants irrigated with wastewater compared with usual irrigated water and it was not translocated from roots to shoots in all treatment, showing the high ability of Vetiver grass to accumulate Hg in roots. The highest and lowest concentration of As and Cr in shoots were 4.19 and 0 (mg kg-1) and 0.15 and 0.0006 (mg kg-1), respectively. Due to negligible translocation of heavy metals, especially Hg, to shoots of vetiver grass, they can be used as alternative forage crops for Animal husbandry in arid and semi-arid regions suffering from water scarcity. The ratio of As in shoots to the total absorbed As in plants was lower compared with literature but this ratios for Cr and Cr were consistent with literature. The order of heavy metal concentrations was as soils>roots>shoots. The information presented herein demonstrates that the vetiver grasses can be grown normally in areas irrigated by industrial wastewater. Results confirmed that vetiver grasses are a green and environmental-friendly system with low-cost maintenances. These grasses have several uses including animal fodder and material for organic farming. Furthermore, the biomass of vetiver grasses can be considered as a promising raw material for biofuel production but needs further investigations for its possibility.