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Zinc and Chromium removal mechanisms from industrial wastewater by using water hyacinth, eicchonia crassipes

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par John Gakwavu Rugigana
National University of Rwanda - Master's in WREM (water resources and environmental management) 2007

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3.1 Water Hyacinth sampling site description

The water Hyacinth plants were collected in Nyabugogo swamp which is located in capital city (Kigali) of the country. The part which is shown as Nyabugogo swamp on the map is one which is not exploited by the population for agriculture and is the one considered as natural wetland, receiving wastewaters of Kigali City. Its surface area, according to CGIS, is 60.09 ha (CGIS, 2007).

Figure 3.1: Topographic map showing the location of Nyabugogo swamp and its influents. (Source: CGIS, 2007)

3.2 Methods

3.2.1 Description

The methodology developed in this research taken the approach consisting in the laboratory pilot scale experiment. Major mechanisms of metal removal were explained on basis of experimental results and available information in the literature review. Adsorption and uptake, translocation and foliar absorption tests were performed to assess the metal removal using water hyacinth. Three replicates were done during the lab experiment.

3.2.2 Synthetic wastewater solution preparation.

1 mol of ZnCl2 contains 1 mol of Zn (II) and also 1 mol of K2Cr2O7 contains 2 mol of Cr(VI). Then knowing that the load of 1 mol ZnCl2 is 136.2 g ,1 mol K2Cr2O7 is 294 g, 1 mol Zn is 65.2 g and 2 mol of Cr is 2* 52 g = 104 g , we calculated the load of each salt to be weighted and dissolved into 1 liter of aqueous solution. Thus for zinc, the calculation has been done as follows: 1g * 136.28 g / 65.2 g = 2.1 g of ZnCl2 and for chromium we did it as follows: 1 g * 294g / 104 g = 2.8 g of K2Cr2O7 .As we decided to prepare 100 ml of solution, 0.21g of ZnCl2 and 0.28 g of K2Cr2O7 were dissolved into 100 ml of solution. For 1 mg/l of Zn (II) and Cr(VI) preparation, we abstracted 1 ml from these 100 ml and dilute up 1000 ml of solution. We did the same for 3 mg/l and 6 mg/l by abstracting respectively 3 ml and 6 ml from the 100 ml of solution and dilute up 1000 ml. The pH of the solution was then adjusted between 6 #177; 0.7 by addition of dilute HNO3 or NaOH as required.

3.2.3 Experimental Procedures

The Water Hyacinth plants (Eichhornia crassipes) were collected from Nyabugogo wetland in Kigali city, were rinsed with tap water and distilled water to remove any epiphytes and insect larvae grown on plants. The plants were placed in big plastic containers with water under natural sunlight for several weeks to let them adapt to the new environment, then the plants were selected and weighted by sensitive balance. The experimental set-up was consisting in the use of small plastic container buckets of 16

cm of diameter and 14.5 cm in height. All experiments were run in a batch system using a nutrient solution constituted by 500 ml of tap water from the valley located at Butare near Pharmacopée centre, 500 ml of wastewater from the Nyabugogo wetland plus quantity of Ca(NO3)2 .4H2O, NaNO3, NH4Cl, K2HPO4 respectively 20, 20, 20 and 40 mg. The fresh weight of the plants in each bucket was measured by using sensitive balance before starting each growing time: 1, 2 and 4 weeks.

A stock solution (1,000 mg/L) of Zn (II) (ZnCl2) and Cr (VI) (K2Cr2O7) was prepared in distilled water, which was later diluted as required. The plants were maintained in water supplemented by Heavy metals by pouring a certain volume of the metals stock solution in order to get the final concentration of 1, 3 and 6 mg/L of Cr and Zn respectively in different plastic buckets containing water hyacinth plant in three replicates.

Plastic buckets with zinc and chromium concentrations without water hyacinth plants served as control. Distilled water was added in order to compensate for water loss through plant transpiration, sampling and evaporation. Water samples were taken and pH measurements by pH meter were taken every 60 minutes for the first day during 6 hours and for the following days one sample after time period was taken during 1, 2, and 4 weeks of exposure to metal solution. All samples were filtered using 0.45 um cellulose acetate filters (wathman papers) and acidified with 5 drops of nitric acid (HNO3 65%) in the laboratory for storage of water samples in volumetric flasks (250 ml) before Atomic Absorption Spectrometer analyses.

The Figure 3.2 shows the plan view of laboratory experimental set up developed during our research in National University of Rwanda, Faculty of Sciences.

Figure 3.2: Plan view of experimental set up.

After each test duration (1, 2 and 4 weeks), final fresh weight for each water hyacinth plant was taken; plants were harvested for other analyses. They were separated into petioles, roots and leaves and were analysed for relative growth, metals accumulation, translocation ability, bioconcentration factor (BCF) and adsorption on the outer surface of roots. For adsorption, roots were immersed in EDTA-Na2 for metal desorption. All parts of the plants were dried in drying oven at 105°C for 24 hours. In addition, the metals remained in the solution were measured to assess the removal potential of water hyacinth plants.

The figure 3.3 depicts different steps developed in the laboratory for data collection and analyses.

Figure 3.3: steps in lab experiment.

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