The usage of polyvinyl and carbon nanotubes modified chitosan for the removal of chromium (VI) from waste water

Nowadays, as the people needs increasing, the industries is needed to have a great growth to meet these needs. However, industrial growth has been resulting with enviromental pollution. Environmental pollution is consist of air pollution, soil pollution, sound pollution and water pollution. In this study, it is focused on water pollution which is led by heavy metal contamination on industial waste water. The heavy metal contamination on industrial waste water cause serious environmental problems. Chromium is known as one of the most abundant ion on industrial waste water. Chromium which is mostly found in the leather, metal, mine and textile industries' waste water, usually exists Cr (III) and Cr (VI) forms. Cr (III) is less harmful compared with Cr (VI), however Cr (VI) has some characteristics threating human life. According to legal regulations, maximum discharge limits of Cr (VI) concentration for some specific industries such as leather, textile, mining and metal, has been defined and industries are responsible to reduce Cr (VI) concentration under legal limitation for their waste water. Recently, there are several technologies for Cr (VI) removal from waste water. Main Cr (VI) removal technologies are membran process, ion exchanging, chemical precitipation and adsorption. Chemical precipitation has disavantage due to high sludge production, ion exchanging, membran processes have a high cost and ion exchanging is needed to high investment cost and relatively complex application. Moreover, adsorption which is one of the most common technology, has some advantages as effective removal, easy applications and relatively low cost. Many adsorbents can be used for adsorption applications. These adsorbents are mainly, zeolites, active carbons, peat, clays and polymer composites. Even though activated carbon which is most widely used as adsorbent has efficient usage, it' s cost is too high comparing with the others. That's why, natural materials such as zeolites, chitosan and clays have more popularity in these days due to their low cost. In this study, polyvinyl alcohol modified chitosan (CS-PVA) and polyvinyl alcohol modified chitosan enhanced by carbon nanotubes (CS-PVA-MWCNT) composite materials in the bead form were readily used as adsorbent. FTIR spectroscopy, TGA, SEM and BET analysis has been studied in order to be construed material characterization to both polyvinyl alcohol modified chitosan and polyvinyl alcohol modified chitosan enhanced by carbon nanotubes. In order to charactrize the functional groups of both composites, FTIR spectroscopy has been analyzed. It has showed that carbon nanotube modification has been successfully executed. TGA graphics has been studied to see the thermal resistance of both composites. Water loss percentages have been measured as 75,35% and 75,21% accordingly for CS-PVA and CS-PVA-MWCNT by using TGA graphics. SEM has been anayzed for better understanding of surface morfology. Surface area of CS-PVA has been determined as 1,63 m2/g and surface area of CS-PVA-MWCNT is 2,5 m2/g with BET anaysis. This has been obliously showed that carbon nanotubes modification has increased surface area in comparision with CS-PVA and CS-PVA-MWCNT composites. The adsorption ambient conditions such as pH, contact time and adsorbent dose have been investigation to undertand how they effect the Cr(VI) adsorption on CS-PVA and CS-PVA-MWCNT. As a result of ambient conditions studies, optimum pH, contact time and adsorbent dose values has been defined seperataly for both CS-PVA and CS-PVA-MWCNT composites. For CS-PVA, optimum pH was defined as 3, adsorbent dose was obtained as 2,5 g/L. Also it has been seen that different initial concentrations of Cr (VI) are needed different contact time. The contact time has been determined as 3 and 16 hours for accordingly 50 ppm and 200 ppm initial concentration of adsorbate. For CS-PVA-MWCNT, optimum ambient conditions are defined as following: pH is 4,5, adsorbent dose is 2,5 g/L and contact time is 1 hour for 50 ppm intial concentration and 16 hours for 200 ppm initial concentration. Kinetic has been studied to understand adsorption reaction mechanism and which diffusion is dominant for adsorption. Kinetic studies has been executed under defined optimum ambient conditions. The theorical isotherm models has been investigated and Langmuir isotherm is defined as best fitting model for Cr (VI) adsorption to both CS-PVA and CS-PVA-MWCNT. Maximum adsorption capacity has been calculated as 54,65 mg/g for CS-PVA and 66,67 mg/g for CS-PVA-MWCNT . It has been defined that rate constant for CS-PVA is first order, for CS-PVA-MWCNT is second order, also both pore and boundry layer diffusion are dominant. Swelling test has been executed to undersand water holding capacity. The results are 46,68% for CS-PVA and 42,64% for CS-PVA-MWCNT. Desorption capacity is also an important factor for adsorbent due to recovery of removed ions and re-usability of adsorbent. That's why, desorption has been studied with 1M HCl, 1 M NaOH and 0,05 M EDTA as desorbent. The best desorbent has been found 1 M NaOH wih the result of 11,2% for CS-PVA and 45,87% for CS-PVA-MWCNT desorption capacity. Since the Cr (VI) solution was prepared in the laboratory, the real industrial waste water is recommended to be studied to understand competing ion effect for adsorption process. The topic is open to study more about ambient conditions such as temperature, particle size and mixing speed to define better optimization of ambient effect. Desorption results can be improved with rejeneration studies to see re-usability of CS-PVA and CS-PVA-MWCNT. This study can help adsorbents be defined more cost effective. This study was executed as batch process, it can also be worked on adsorption column filled with CS-PVA and CS-PVA-MWCNT to make the study more match with industrial facts. As a conclusion, this study shows that multi-walled carbon nanotubes modification on chitosan/polyvinyl alcohol compaunds improves the adsorption capacity with optimized ambient conditions such as contact time, pH and adsorbent dose. Also, the characterization analysis have supported these results.