Phytoremediation, which is an emerging technology for cleaning up contaminated sites, is cost effective and has aesthetic advantages and long term applicability. The technology involves efficient use of plant species to remove, detoxify or immobilize contaminants in a growth matrix (soil, water or sediments) through natural processes. For this study, swamp smartweed (Polygonum coccineum), para grass (Brachiara mutica) and papyrus (Cyprus papyrus) were grown on 20 mm crushed rock filled plastic vessel watered with synthetic chromium containing solution in a greenhouse under ambient conditions. For comparison, the plants were also grown on both synthetic solution and tannery effluent with known concentration of 0.1, 0.5 and 1 mg/L Cr3+ at Bahir Dar tannery. Plants were harvested after 72 days of planting period and their roots and above ground parts were cleaned and digested through microwave digester for further analysis. Chromium in roots and shoots was determined by ICP-OES. It was observed that all plant species have the ability to remove both Cr3+ and Cr6+ from the aqueous solution for the specified initial concentration. Interestingly, using single factor analysis of variance, significant differences were also observed in their partitioning. All the three plants exhibited a significant transfer of Cr from wastewater to roots and shoots, but removal efficiency of Cr for swamp Smartweed was relatively low as compared to para grass and papyrus. On average translocation factor and removal efficiency of para grass for synthetic solution at 0.5 mg/L level and papyrus for tannery effluent at 1 mg/L level of chromium were relatively high (1.260, 83.08% and 1.715, 73.77% respectively). This finding indicated that all the tested plants (swamp smartweed, para grass and papyrus) can be used for phytoremediation of Cr3+ and Cr6+ containing wastewater discharged from industries.
Industrialization is the key for the development of the society. On the other hand, it is a major contributor to environmental pollution. When thinking of development we should think of its effects on the ecosystem. Heavy metal release from industries is the major problem to the environment and living things. Metal contaminated soil and water can be remediated by chemical, physical and biological techniques (Ghosh 2005). Recently, researchers are working on the environmental remediation of heavy metals by the natural ways rather than the chemical methods (Akpor & Muchie 2010; Tangahu et al. 2011).
Water is one of the most important natural resources, and it is essential for all forms of life. This natural resource is being contaminated every day by various activities such as rapid population growth, urbanization and industrialization that ultimately make the environment polluted. Since recent years, sewage waters have been being used for irrigation purposes. The occurrence of toxic heavy metals in the soil is of geogenic or anthropogenic origin (Singanan et al. 2006). Heavy metals can be transported from the point of origin and other sources to distant environments High levels of heavy metals can damage soil fertility and may affect productivity. Heavy metals in the environment may also change plant diversity and affect aquatic life.
Chromium (III) in trace concentration is essential in the diet, because it regulates the glucose metabolism in the human body (Anderson 1986; Ali & Sajad 2013). However, excess amounts of chromium uptake are very dangerous due to its carcinogenic effect. Chromium in soils affects plant growth. It is non-essential for microorganisms and other life forms and when in excess amounts it exerts toxic effect on them after cellular uptake (Ali & Sajad 2013). Cr (VI) is more toxic than Cr (III). Leather and chromium plating industries are the major causes for environmental influx of chromium. The movement of chromium and its bioavailability poses a potential threat to the environment.
Phytoremediation is one of the ways to solve the problem of heavy metal pollution using plants (Bennicelli et al. 2004; Mohebbi et al. 2012; Balabanova et al. 2015). In the process of phytoremediation pollutants are collected by plant roots and either decomposed to less harmful forms or accumulated in the plant tissues. Thus, phytoremediation is environmentally friendly, inexpensive and can be carried out in polluted places (remediation in situ) plus the products of decomposition do not require further utilization (Bennicelli et al. 2004).
Ethiopia is one of the naturally gifted countries with a variety of endemic plant species. This research is conducted on three plants (papyrus- Cyprus papyrus, smartweed- Polygonum coccineum, and para grass- Brachiara mutica). Those plants are local to Ethiopia and grow on the shore of Lake Tana which is the biggest lake in the country. Thus, the aim of this study is to conduct an experiment on local plants and to suggest them for remediation of toxic chromium containing tannery wastewater in the environment.
MATERIALS AND METHODS
Materials and chemicals
Analytical grade chromium salts, CrCl3.6H2O and K2Cr2O7 (Blulux laboratories (P) ltd, India) and distilled water were used to prepare feed solutions to the plants. Three emergent aquatic vascular plant species from Lake Tana were chosen; (papyrus – Cyprus papyrus, swamp smartweed – Polygonum coccineum, and para grass – Brachiara mutica) for experiment to be subjected to Cr3+ and Cr6+ treatments. The experiments with synthetic solution were performed at Bahir Dar University in a green house and experiments with tannery effluents were conducted at Bahir Dar Tannery Enterprise.
The experiments were carried out on plants grown in 16 L plastic pots half filled with 20 mm sand and tap water. The selected plant species were planted in the pots and placed in a green house. Before the experiment the plants were irrigated with tap water for adaptation. Synthetic solutions containing Cr3+ and Cr6+ were prepared from CrCl3.6H2O and K2Cr2O7, respectively. This experiment was also done with tannery effluent in an ambient air condition.
The concentrations of chromium ions added were 0.1 mg, 0.5 mg and 1 mg per liter. Six levels of chromium treatments for the synthetic experiments were considered: three for Cr (III) and three for Cr (VI. And three levels of chromium treatment for tannery effluent (only chromium three ions) plus a control for each plant were conducted. There were two seedlings per container within each container as replicate. Young plants were detached from mother plants, washed and planted in the prepared pots. Essential macronutrients P, N and K were supplied to the plants using KH2PO4, KNO3 and KH2SO4 aqueous salt solutions, respectively .The micronutrient, Fe was supplied using iron sulfate salt solution (Shive 1915). Both macronutrients and micronutrients were added once a week to grow the plants well. After one month, addition of known concentrations of chromium was started .The experiment continued for 42 days; finally the mature plants were uprooted and separated into roots, stem and leaves. These parts were kept in a dry condition for the analysis of accumulated and translocated chromium ions.
Sample preparation and analysis
The parts of plants (leave, stem and root) were separated and thoroughly washed with distilled water to remove ions and dusts. The plant samples were dried in a convection oven (PH-030A) for 48 hours at 80 °C. Dry shoots and roots were ground with mortar and pestle and weighed with digital mass balance. Plant samples from each part (0.5 g) were digested with 10 mL concentrated 65% HNO3 and 2 mL H2O2 in microwave digester for 30 minutes and cooled for 15 minutes (Fred et al. 2001; Narain et al. 2011; Ali & Sajad 2012). The digest was allowed to cool and then filtered through a Whatman filter paper grade 541(22 μm pore size). The filtrate was collected in a 50 mL volumetric flask and diluted to the mark with distilled water. The filtrate was used for the analysis of chromium metal by using inductively coupled plasma-optical emission spectrometry (Horiba Jobin Yvon, ‘ULTIMA-2’) using argon/air as gas mixture. The wave length (λ) was 267.716 nm. Each sample was analyzed in triplicate.
Determination of total chromium in wastewater sample
To determine total chromium in the wastewater, first the samples were digested using concentrated nitric acid following standard procedures for the examination of water and wastewater (APHA 1998). Finally the samples were filtered through Whatman filter paper grade 541, England and the chromium concentration was determined using Ultima 2 Inductively Coupled plasma Optical Emission Spectrometry(ICP-OES), Horiba Scientific.
Bioaccumulation factor and translocation factor
Statistical data analysis
Each experiment was conducted in triplicate (n = 3). Results were shown as mean ± standard error. One-way analysis of variance (ANOVA) was used to interpret the result using SPSS 20.0 statistical software
RESULTS AND DISCUSSION
The removal of Cr+3 and Cr+6 ions by different tissues (stems, leaves and roots) of papyrus- Cyprus papyrus, swamp smartweed- Polygonum coccineum, and para grass- Brachiara mutica was shown in Figure 1 below.
As it can be seen in Figure 1(a) and 1(b), all plants have a capacity to uptake chromium from the synthetic solution. The plants accumulated relatively high amounts of Cr3+ and Cr6+ at low level of initial chromium concentration. All the plants preferentially uptake more Cr6+ as compared to Cr3+ in synthetic solution. It is reported that Cr6+ is more mobile and toxic than Cr3+, and there is no evidence of the potential role in plant metabolism (Panda & Patra 1997). Singh (2001) reported 3.14 and 2.8 μg g−1 of Cr6+ and Cr3+, respectively were accumulated in the plant of spinach. It was observed that para grass accumulated relatively high concentrations of Cr3+ and Cr6+ in their tissues for the three levels of chromium concentration as compared to papyrus and smartweed. The total accumulation of both chromium species was higher in the roots as compared to the stem and leaves of the plants. Especially Smartweed accumulated the highest amount of Cr6+ in its roots compared to para grass and papyrus. Studies indicated that large amount of chromium taken up by aquatic and terrestrial weeds accumulated mainly in the roots than the shoots (Paiva et al. 2009; Sundaramoorthy et al. 2010). Cr (VI) is actively taken up and is a metabolically driven process in contrast to Cr (III) which is passively taken up and retained by cation exchange sites of the cell wall (Shanker et al. 2004). This in part explains the higher accumulation of Cr (VI) by the plants. In addition, Cr (VI) competes with P for surface sites and Fe, S and Mn for transport binding (Wallace et al. 1976); hence, it seems that Cr (VI) has an advantage at the entry level into the plant system. However, it should be noted that Cr (III) can easily enter the system if it is organically complexed at the rhizosphere level (Shanker et al. 2005).
Similarly, experiments for Cr+3 were performed using tannery effluent and the result is presented in Figure 2 below. At lower chromium concentration, the three plants indicated good accumulation of Cr3+. Through the experimental period, papyrus accumulated higher chromium ion (6.45 mg/kg) than both smartweed and para grass species. As the chromium concentration increased the capacity of uptake of Cr3+ from the tannery effluent decreased. In the tannery effluent within this range of concentration, it produced a good condition for papyrus to grow well and took high concentration of Cr3+ compared to para grass and smart weed. It has been found that ability of Cr to form complexes with organic ligand increases its uptake in plants (Srivastava et al. 1999).
Translocation factor and bioaccumulation factor
The TFs generally showed the movement of chromium from root to shoot, indicating the efficiency to uptake the bio-available metals from the system. The species of plants selected for chromium removal were all efficient to take up and translocate chromium from root to shoot (Figure 3) with noticeable variations between TF values. The highest TF values (3.271) of Cr in tannery effluent in level 3 and 3.1346 in level 2 were observed for smartweed and 3.08 for para grass in level 3 experiments. In the synthetic solution experiment in both level 2 and level 3 of Cr3+ and Cr6+ para grass showed TF of 2.477 and 2.359 respectively. According to Ghosh (2005) and Badr (2012), high root to shoot translocation of heavy metals indicated in phytoextraction of heavy metals. It is easy for plant species with TF > 1 to translocate metals from roots to shoots than those which restrict metals in their roots.
The phytoextraction efficiency of Cr+3 and Cr+6 in different parts of the three plants was calculated using BAF using Equation (1) above. The three plant species relatively accumulate higher concentrations of both chromium species in their roots than stems and leaves. This is due to immobilization of both chromium ions in the vacuoles of the root cells (Shanker et al. 2004). Smartweed accumulated highest amount of chromium in its roots in the tannery effluent experiment. A study conducted in situ to assess the phytoremediation of Cr(VI) using para grass (Brachiaria mutica) at South Kaliapani chromite mine area in Orissa state, India, the BAFs of Cr in para grass (Brachiaria mutica) weed grown in 100 days was 0.334 (Mohanty & Patra 2012).
The synthetic solution experiments indicated that Para grass was efficient in removing both Cr3+ and Cr6+ with removal efficiency of 79.13% and 83.08% respectively. But it showed low removal efficiency in real tannery effluent. Papyrus showed better removal efficiency of chromium both in synthetic and tannery effluent, as shown in Figure 4 below. The study indicated Smartweed prefers Cr6+ than Cr3+. Smartweed showed low removal efficiency (17%) for Cr3+ ions in synthetic solution compared to other two plants. Generally Cr6+ was better removed by the plants compared to Cr3+ in both synthetic and tannery effluent (Figure 4).
A one-way ANOVA between groups was conducted to explore three different plant species (smartweed, para grass and papyrus) removal efficiencies of Cr (III) and Cr (VI) at their roots, stems and leaves. There was a statistically significant difference at the p < 0.05 for Cr (III) and P < 0.001 for Cr (VI) levels in the roots, stems and leaves of these plants in synthetic solutions. In the real tannery wastewater the removal efficiency of Cr (III) in these plant parts was also significant at p < 0.05
The study showed that the plants were promising for removal of both chromium species from wastewater. Para grass (Brachiara mutica) and papyrus (Cyprus papyrus) showed relatively better removal efficiencies. The removal efficiency of swamp smartweed (Polygonum coccineum), Para grass (Brachiara mutica) and papyrus (Cyprus papyrus) of Cr3+, Cr6+ (synthetic solution) and chromium in tannery effluent was (17%, 81.4%, 51.4%), (79.3%, 83.08%, 32%), and (76.37%, 67.4%, 73.77) respectively.
Generally this finding showed that all the plants (swamp smartweed, para grass and papyrus) have the potential to remove chromium from wastewater polluted from industries. The BAFs and TFs of the three plants showed that they can be used for phytoremediation of chromium, especially with level 2 (0.5 mg/L) and level 3 (1 mg/L) chromium and they also showed TF > 1.They are highly promising for remediation chromium containing wastewater. In the case of tannery effluent papyrus showed high removal efficiency, so we can say papyrus is very efficient in removing Cr3+ in both the synthetic and tannery effluent experiments.