2.1 Collection of Banana Rachis Banana rachis waste was collected from a nearby banana market at Khulna, Bangladesh and was burnt to prepare charcoal for wastewater treatment.
2.2 Charcoal Preparation After collection of banana rachis waste, it was washed and then cut into small pieces and was sun-dried. Then the dried pieces of banana rachis were burnt in aerobic condition, cooled and ground to make a powder using a mortar. The ground charcoal was sieved and preserved for the experiment.
2.3 Wastewater Collection Chromium-containing wastewater was collected from a local tannery at Khulna, Bangladesh. The wastewater containing chromium was collected in a plastic container pre-washed with diluted nitric acid and rinsed with distilled water. It was immediately transported to the laboratory for experiments.
2.4 Reagents Nitric acid (Merck KGaA, Germany), sulphuric acid (Merck KGaA, Germany), perchloric acid (Merck, India), N-phenyl anthranilic acid (Merck, India), ferrous ammonium sulphate (Merck, India) and glass beads (Loba Chemie, India) were purchased from a local scientific store, Khulna, Bangladesh.
2.5 Characterization of Tanning Wastewater Physicochemical properties of wastewater were measured in terms of chromium, pH, total dissolved solids (TDS), electrical conductivity (EC), salinity, dissolved oxygen (DO), biochemical oxygen demand for 5 days (BOD5), and chemical oxygen demand (COD).
2.5.1 Determination of pH The pH of the spent chrome liquor was measured by using the pH (UPH-314, UNILAB, USA) meter. Before measuring the pH meter was calibrated with the standard buffer solution.
2.5.2 Determination of TS, TSS, TDS, and EC TS, TSS, and TDS of the spent chrome liquor and treated liquor were determined by APHA2540 D method. A well-mixed sample was filtered through a weighed standard filter paper and the residue retained on the filter was dried to a constant weight at 103-105°C. The increase in weight of the filter represents the total suspended solids. To obtain an estimate of total suspended solids, the difference between total dissolved solids and total solids was calculated. Electrical conductivity (EC) was measured by using the conductivity meter (CT676, BOECO, Germany).
2.5.3 Biochemical Oxygen Demand: Biochemical oxygen demand of the spent chrome liquor and treated liquor were determined by APHA-5210 B method. The sample was filled in an airtight bottle and was incubated at a specific temperature for 5 days. The dissolved oxygen (DO) content was determined before and after five days of incubation at 20°C and the BOD were calculated from the difference between initial and final DO.
2.5.4 Chemical Oxygen Demand: Chemical oxygen demand of the spent chrome liquor and the treated liquor were determined by APHA-5220 C method. The organic matter present in sample gets oxidized completely by potassium dichromate (K2Cr2O7) in the presence of sulphuric acid (H2SO4), silver sulfate (Ag2SO4) and mercury sulfate (HgSO4) to produce CO2 and H2O. The sample was refluxed with a known amount of potassium dichromate (K2Cr2O7) in the sulfuric acid medium and the excess potassium dichromate (K2Cr2O7) was determined by titration against ferrous ammonium sulfate, using ferroin as an indicator. The dichromate consumed by the sample is equivalent to the amount of O2 required to oxidize the organic matter.
2.5.5 Chloride Content 50 mL of the sample in a beaker with 5 drops (about 1mL) of potassium chromate indicator was titrated by standard (0.0141 N) silver nitrate solution from a burette, with constant stirring until the first permanent reddish color appeared. Chloride content was determined from the required amount of silver nitrate comparing with distilled water for blank following APHA-4500B method. If more than 7-8 mL of silver nitrate solution is required, the entire procedure should be repeated using a smaller sample diluted to 50 mL of distilled water.
2.5.6 Determination of Chromium Chromium content in the untreated spent liquor and after treatment in the filtrate was measured by the titrimetric method by following the official methods of analysis of Society of Leather Technologist and Chemists (1996) official method of analysis SLC 208 (SLT6/4). A 50 mL sample was taken in 500 mL conical flask. 20 mL concentrated nitric acid was added followed by 20 mL perchloric acid/sulphuric acid mixture; the flask was gently heated and boiled until the mixture had become a pure orange-red color and continue boiling for one minute. The flask was removed from the heating source and as soon as ebullition has ceased; rapidly the flask was cooled by swirling in cold water bath. Carefully, 100 mL distilled water was added with a few glass beads and boiled for 10 minutes to remove free chlorine. Then, 10 mL 30% (v/v) sulphuric acid was added and cooled to room temperature. The mixture was titrated with freshly prepared 0.1N ferrous ammonium sulfate solution with six drops of N-phenyl anthranilic acid as an indicator. The end color was indicated by a color change from the violet to green.
2.6 Treatment of Chromium Containing Wastewater Batch-wise chromium removal examination was performed with the prepared charcoal. Firstly, physicochemical parameters of the untreated chromium-containing wastewater were analyzed and filtered through 0.45 µm pore size filter. Secondly, 75 mL filtrate wastewater was mixed the prepared charcoal. The charcoal mixed wastewater was stirred over a fixed period of time and the mixture was then allowed for settling for a fixed time. After settling, the mixture was filtered through 0.45 µm pore size filter again chromium content measurement was performed the same procedure as described in section 2.5.6.