Removal of chromium (VI) from aqueous solution by the modified Tamarix sawdust: Batch and column studies

Nowadays, removing pollutants from water and wastewater is of utmost importance. The pollution of water resources due to indiscriminate disposal of heavy metals has been causing a worldwide concern for the last few decades. Chromium is one of the heavy metals which is found in weaving, currier and fertilizer factories. Many methods of treatment for industrial wastewaters have been reported in the literature. Amongst these methods, the most popular are neutralization, precipitation, ion exchange and adsorption. For low concentrations of metal ions in wastewaters, the adsorption process is recommended. The process of adsorption implies the presence of an “adsorbent” solid that binds molecules by physical attractive forces, ion exchange and chemical binding. It is advisable that the adsorbent is available in large quantities, easily regenerable and cheap. Usage of natural adsorbents as replacements for imported chemical adsorbents is important for developing countries. theapplication of several adsorbents such as Wheat bran, Sawdust, Tea, Modified Sand and Activated Carbon prepared were reported in the literature. Tamarisk is the only tree which can grow in Sistan plain and its sawdust is abundantly found in factories. The purpose of this research is to use of local adsorbent modified Tamarix sawdust for removal of hexavalent chromium from aqueous solution. In practice obtained adsorption from discontinuous studies capacity may not represent accurate information in pilot scale in a column system. Therefore, continuous studies in column system are required to estimate breakthrough curve of the column, determination of breakthrough point and evacuation point of the column. Several mathematical models are represented to design adsorbent columns such as BDST, which is extensively has been used in literature. Batch sorption was studied to determine optimal pH, equilibrium time and isotherm model parameters. In adsorption isotherm experiments 1-20 gr/l of adsorbents and 8 mg/l of Cr(VI) solution were reached to 100 ml. Isotherm models such as Freundlich and Langmuir were used to fit to experimental data in non-linear form of the equations. Also for removal of chromium in continuous condition a 30 cm bed depth glass column with diameter of 3 cm was used as a fixed bed with gravity water flow. Operation of the column was investigated under flow of 5, 6.5 and 8 ml/min, bed depth of 10, 12 and 14 cm and Cr(VI) concentration of 6, 8 and 10 mg/L. Finally, the absorber column behavior using BDST, was studied and model parameters were determined.
Adsorption efficiency of Cr(VI) decreases with increase in pH. Minimum adsorption efficiency occurs at pH=5, therefore all the experiments were done at optimal pH (pH=1). The predominant form of Cr(VI) is HCrO4- at pH values lower than 1, while the adsorbent surface is ready to adsorb. However, at higher pH values predominant form of Cr(VI) is as H¬2CrO4 and CrO3 forms, which have no tension to react. As a result OH- will be adsorbed more, therefore the most adsorption of Cr(VI) occurs at pH=1. With increase in contact time, Cr(VI) adsorption increases for a fixed mass of adsorbent, as in 0-30 min adsorption rate is very high.
The results of this research showed that maximum adsorption efficiency by modified sawdust occurred in equilibrium time of 120 min. The Langmuir and Freundlich models were used to explain the equilibrium data. The Freundlich model showed better fit to the data with a correlation coefficient of 0.9637. Also column studies show that the time to reach the breakthrough point increases with increase in bed depth. With increase in bed depth, gradient of breakthrough curve decreases. As a result, mass transfer zone expands and it takes more times to effluent. Also with increase in flow rate and initial concentration, gradient of breakthrough curves is more and leads to reduce the breakthrough time. On the other hand, the adsorption capacity increases with increase in bed depth and decrease in flow rate and initial concentration. The breakthrough time of the column decreases with increasing the flow rate and initial concentration, but it increases with increase in bed depth. The results showed that surface adsorption of Cr(VI) is highly dependent on flow rate, initial concentration and bed depth. Evaluation of column parameters showed that BDST model has a good concordance with experimental data.parameters showed that BDST model has a good concordance with experimental data.