Reagents
Potassium dichromate (Fisher Scientific; Pittsburgh, PA, USA) was used as a source of hexavalent chromium, diphenylcarbazide (Merck Germany), sulfuric acid (98%), hydrochloric acid (85%) and sodium hydroxide were all of analytical reagent grade and obtained from Sigma (St. Louis, MO, USA). Deionized water was used for all solutions and dilutions.
Extraction of Tamarind Leaves
Tamarind leaves were collected by a research assistant from Pharmacognosy Research Laboratory, Ahmadu Bello University Zaria. The leaves were washed with clean water and dry before ground into fine powder. Extraction was carried out by maceration through soaking 500g leaves powder in 70% methanol for two days. The solution was decanted and filtered through Whatman No.4 filter paper. The mixture was concentrated to a thick consistency and the resulting extract kept in a desiccator for further use.
Preparation of Reagents
The extract solution was prepared by dissolving 0.1g in deionized water and subsequently diluted to 100mL to obtain a 1mg/ml extract solution. Stock Cr (VI) solution (50 mg L − 1) was prepared by dissolving 0.05 g K2Cr2O7 (294.18 g mol − 1) in1 L of deionized water. The pH of Cr(VI) solution was adjusted to 7.0 using 0.1 M NaOH or 0.1 M HCl before filtered using a 0.45-µmWhatman filter paper and sterilized. The working solution was prepared by diluting the stock solution with deionize water to give the appropriate concentration (10 mg L–1) of the solution and 0.20 g of 1,5 Diphenylcarbazide was added in 100ml of 95% acidified ethanol and store in sterilized and dried brown colored bottle.
Collection of Sample
Contaminated soil and tannery effluent were collected from Nigerian Institute of Leather Science and Technology (NILEST). While, uncontaminated soil sample was collected from a random location and thereafter identified as sandy soil with the following characteristics of < 18% clay and > 68% sand, weak structure, 33–47% porosity, 25% field capacity, pH of 6.7, 95% hydraulic conductivity, 45% infiltration rate and bulk density of 1.6g/cc are commonly recorded22. Loose particles and plant debris were removed manually, the sample were ground into fine powder and passed through 0.149mm sieve 23. Normal tap was used to prepare a 5mg/L Cr(VI) solution for simulation of contaminated water.
Determination of Cr(VI) Concentration
The experiment for the reduction of Cr (VI) was conducted as described by Chen et al.16 with some modifications. The reaction mixtures were obtained by adding a source of Cr(VI) into a 250mL Erlenmeyer flask, and Tamarindus indica methanolic leaves extract or equivalent amount of particles. The initial pH of the solution was adjusted with sulfuric acid solution (0.5M) and/or sodium hydroxide solution (1.0M). All experiments were conducted at room temperature (25oC) unless otherwise specified. Hexavalent chromium was measured spectrophotometrically at 540nm using 1, 5 - diphenylcarbazide.
Preparation of Extract-immobilized Microparticles
Guinea corn (Sorghum bicolor) stalk was obtained from a farm, dried and grounded into powder. The microparticles were prepared by simple adsorption through mixing the extract solution with lignocellulosic powder as described by Sharma et al.24 with little modifications. About 1g of lignocellulose powder was dissolved in 100ml of 1mg/ml extract and the solvent allowed to evaporate at 40oC, while the extract absorbed into the powder. The samples were kept in an airtight container at 4oC until needed.
Characterization of Microparticles
The size and morphology of the microparticles produced were determined according to the method described by Harris et al.25, with the aid of Scanning Electron Microscopy (SEM) Analysis. The particles were mounted on metal stubs with double-side adhesive, and coated with gold in vacuum. The morphology was viewed on the screen and then captured at different magnifications.
Microparticles Adsorption Efficiency
Tamarind leaves extract adsorption efficiency was carried out according to Harris et al.25. Extract adsorbed in microparticles was quantified by Folin–Ciocalteu method with respect to total phenolic content by dissolving 30 mg of the particles in 20 mL of deionized water. The experiments were carried out in triplicate to minimize error.
Kinetic Release Studies on Microparticles
Tamarindus indica methanolic leaves extract release kinetics was determined according to Harris et al.25. Microparticles (150 mg) were suspended in deionized water at 37 ◦C and shake at 100 rpm (Rotabit horizontal shaker, Selecta, Barcelona, Spain). The suspension was aliquoted in 1 mL tubes after different time intervals, centrifuged at 15,000 rpm for 30 min. One milliliter of deionized water was added into the suspension to replace the amount taken in each step. The release of the extract was quantified using Folin–Ciocalteu method and the experiments were carried out in triplicate.
Determination of Cr(VI) Reduction by Microparticles in Contaminated Water Model
Cr(VI) reduction capacity of microparticles in contaminated water was determined based on the method reported by Kassama and Misri,26. Accurately weighed extract loaded microparticles equivalent to ~ 1mg and 0.5mg of free extract and empty particles as control were placed in a predetermined 20ml of Cr(VI) solution (5mg/L). The system was maintained at a predetermined condition of room temperature, acidic and neutral pH, and time. The aliquots of 1.5ml were collected after shaking the dissolution flask at regular time intervals (1-4hours) and residual Cr(VI) quantified using 1, 5- diphenylcarbazide method. The absorbance was measured at 540nm by using a UV spectrophotometer.
Determination of Cr(VI) Reduction by Microparticles in Contaminated Soil Model
The method described by Okello et al.27 was used to determine the reduction of Cr(VI) in contaminated soil model. Accurately, 3 g of finely ground soil was mixed with 6 ml of Cr(VI) solution (5 mg/L) in an Erlenmeyer flask and kept for 2 hours prior to the reduction studies. Appropriate weight of the microparticles corresponding to 1mg and 0.5 tamarind extract were added to the mixture and shake. At different time intervals an aliquot of 1mL was withdrawn and added 9 mL of deionized water and centrifuged at 3500xg for 10 minute, 1mL supernatant was assayed for residual Cr (VI) concentration. This same procedure was also used on empty microparticles as a control.
Determination of Cr(VI) Reduction in Tannery Effluent
Chromium (VI) reduction capacity of microparticles in tannery effluent was determined based on the method reported by Kassama and Misri,26. Accurately weighed extract loaded microparticles equivalent to ~ 1mg and 0.5mg of extract and empty particles as control were placed in 20ml tannery effluent in a separate Erlenmeyer flask. The system was maintained at a predetermined condition of room temperature, acidic and neutral pH and time. The aliquots of 1.5ml were collected after shaking the dissolution flask at regular time intervals (1-4hours) and quantified for residual Cr(VI) concentration. This same procedure was repeated using empty microparticles as a control.
Determination of Cr(VI) Reduction in Contaminated Soil
The method described by Okello et al.27 was used to determine the reduction of Cr(VI) in contaminated soil. Accurately, 3 g of finely ground contaminated soil was placed in an Erlenmeyer flask and kept for two days prior to the reduction studies. Appropriate weight of the microparticles corresponding to 1mg and 0.5 tamarind extract were added to the mixture and shake. At different time intervals an aliquot of 1mL was withdrawn and added 9 mL of deionized water and centrifuged at 3500xg for 10 minute, 1mL supernatant was assayed for residual Cr (VI) concentration. This same procedure was repeated using empty microparticles as a control.
Statistical Analysis
All the experiments were carried out in triplicate and the results presented as mean ± SD.