A study was conducted to find out the optimum parameters such as contact time, initial concentration, pH, and adsorbent dose for removing hexavalent chromium from the wastewater of the textile and leather industries by the use of biodegradable composites made from the combinations of Banana and Areca fibers treated with Chitosan.
The study was conducted by Professor Dr. Hosne Ara Begum, Department of Yarn Engineering, Bangladesh University of Textiles and A. K. M. Monjurul Haque Nirjhar, Superintendent, Ministry of Textiles and Jute, Government of the Peopole’s Republic of Bangladesh at Textile Vocational Institute, Cumilla along with some other authors.
The study found that the optimum pH range for best adsorption of the chromium by these biodegradable composites is 2.5 to 4.5 with a contact time of 30 to 180 minutes. Various combinations of Chitosan with Banana and Areca were used in the experiments in a 30:70, 50:50, and 70:30 ratio to find the optimum one. The optimum adsorption of Chromium capacity was recorded with the composite of Chitosan and Banana at a 70:30 ratio, with a pH of 4 and a contact time of 180 minutes.
Bangladesh is heading towards becoming a fashion sourcing hub with the significant improvement of its textile and apparel industry. The rising textile, apparel, and leather industries are also responsible for the drastic pollution of the environment. Especially, water pollution caused by the wastewater of the textile and tannery which contains heavy metals like Chromium has become a major concern for the health-safety of local people. Among various chemicals used in the textile and leather industry, this Chromium is a dangerous substance that is responsible for damaging our liver, kidney, and nervous system, and causes irregular heart rhythms. In the plants, it affects the germination, shoot growth, leaf growth, and production of fruits and flowers. Hexavalent chromium is so toxic that even its low concentration in drinking water and food could be fatal to animals and plants.
These reasons create an appeal of removing this harmful substance from the wastewater generated from the textile and tannery industries so that it can not be accumulated in animals and plants. Various methods can be used to remove heavy metal ions like this Chromium such as chemical precipitation, ion exchange, adsorption, membrane filtration, electrochemical treatment technologies, bio-sorption, etc.
Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. It is now recognized as an effective and economic method for removing heavy metals from wastewater. In addition, adsorption is sometimes reversible and adsorbents can be regenerated by removing the metals from the adsorbents with suitable desorption processes.
Chitosan is a very effective substance to capture the Chromium from a solution. It has been found that in an acidic pH condition, preferably below pH 3, about 90% of the metals can be removed from a solution regardless of the initial concentration and adsorbent dose. Cr (VI) exists predominantly as HCrO4– in an aqueous solution at a pH of below 4. As the amino groups of Chitosan are protonated at this pH, electrostatic interaction occurs between the Chitosan and HCrO4- ions resulting in a high chromium removal from the aqueous solution. Banana and Areca fibers treated with Chitosan can also capture Chromium from an aqueous solution.
Raw materials and chemicals used in these experiments are Prawn shells (for Chitosan), Banana fiber, Areca fiber, Sodium hydroxide, Acids, Oxidizing agents, and Water.
Digestion of sample:
For the quantitative analysis of Cr, samples were digested following the aqua regain extraction process recommended by the International Organization for Standardization (ISO).
Total heavy metals concentration measurement:
After digestion, the samples were analyzed for metals Cr with a Perkin-Elmer atomic absorption spectrometer (Model-An Analyst 800, USA). A specific hollow cathode lamp was used to determination of Cr at 357.9 nm wavelength.
Preparation of composites:
To prepare the composite of Chitosan (CH) and fiber 1% (w/v) Chitosan solution was prepared by dissolving Chitosan in a 1% (v/v) aqueous acetic acid solution and stirred with a magnetic stirrer for 30 minutes to dissolve Chitosan. Then 1gm fiber was added to the Chitosan solution. After that, the fiber was separated from the Chitosan solution and oven-dried for 24 hours at 60°C. Various compositions of the fibers and the Chitosan were made as follows:
Various batch experiments were carried out to evaluate the influence of pH, contact time, and adsorbent dose on the removal of Chromium from an aqueous solution.
The amount of Chromium adsorbed per unit of adsorbent powder was calculated according to the following equations.
Here, Q is the adsorption capacity metal ion absorbed per gram of adsorbent, Co and Ce is the initial and final concentration respectively, M is the mass of the adsorbent (in mg) and v is the volume of chromium solution taken for the adsorption study (in litter).
Results and discussion
From the experiments it was found that the untreated Banana and Areca fibers’ optimum adsorption of Chromium was 3.65 ppm (9.1%) at 180 min, pH of 4.5 and 140 mg adsorbent dose size and 3.76 ppm (9.4%) at 180 min, pH of 4.5 and 160 mg adsorbent dose size respectively. In the case of Chitosan-treated Banana fiber, it increased for 30:70 combination to 28.785 ppm, for 50:50 combination to 30.057 ppm, and for 70:30 combination to 31.574 ppm. For Chitosan treated Areca fiber, it increased, for 30:70 combination to 29.512 ppm, for 50:50 combination to 30.156 ppm, and 70:30 combination to 32.074 ppm. Optimum Chromium adsorption has been recorded at 110.7 mg/g Chitosan: Banana (70:30) at the pH of 4 and contact time of 180 minutes.
The optimum pH, contact time, and adsorbent dose has been found from the test results of the samples developed in the experiments. The ratio of Chitosan and Banana or Areca in the samples were respectively 30:70, 50:50, and 70:30.
All the three combinations of Chitosan with the Banana and Areca fibers show the highest adsorption in the pH range of 3.5 to 4.5 in the test results.
All the three combinations of Chitosan with the Banana and Areca fibers show the highest contact time range of 160 to 180 minutes in the test results.
All the three combinations of Chitosan with the Banana and Areca fibers show the highest adsorbent dose range of 140 to 160 gm in the test results.
As these biodegradable combinations of Chitosan-treated Banana and Areca fibers have demonstrated successful removal of Chromium from the aqueous solution they can also be used to treat textile and leather industry wastewater. And further studies should be carried out to measure the effectiveness of the same combinations to remove other toxic heavy metals like Pb, Cd, As, and Cu.