Chromium (III) Removal and Recovery from Tannery Wastewater by Precipitation Process

Chromium (III) salts are the most widely used chemi cals for tanning processes, but 60-70% of total chromium salts reacts with the hides. In t he other word, about 30-40% of the chromium amount remains in the solids and liquid wastes (esp ecially spent tanning solutions). Therefore, the removal and recovery of the chromium content of the se wastewaters are necessary for environmental protection and economic reasons. Removal and recove ry of chromium were carried out by using precipitation process. For this purpose, three prec i itating agents calcium hydroxide, sodium hydroxid e and magnesium oxide were used. The effects of pH, s tirring time, settling rate and sludge volume were studied in batch experiments. Results show that the optimum pH is 8-9 and the good sludge with high settling rate and lower volume obtain by the MgO pr ecipitating agent. Hence the MgO is a good precipitating agent for removal and recovery of chr omium from tanning wastewater.


INTRODUCTION
Chromium solutions are widely used in many industrial processes such as chrome plating, wood preserving, textile dyeing, pigmenting, Cr chemical production, pulp and paper industrial and tanning. The wastewater resulting from these processes contains high amount of chromium metal which is harmful for the environment and human health [1,2] .
Tanning process using chromium compounds is one of the most common methods for processing of hides [3] . In this process about 60% -70% of chromium reacts with the hides. In the other word, about 30%-40% of the chromium amount remains in the solid and liquid wastes (especially spent tanning solutions). Hence, the wastewater of the tanning process is an important source adding Cr pollutant to the environment. In addition, the cost of the chromium metal is also important and it is possible to be recovered from the wastewatert [4][5][6] . Hafez et al. [7] and Chaudry et al . [8] reported that the Cr ion concentration in the tanning wastewater varies from 2500 to 8000 ppm and 1300 to 2500 ppm, respectively [7,8] .
Several methods have been used for removing toxic metal ions from aqueous solutions. These include chemical precipitation, ion exchange, reverse osmosis, membrane processes, evaporation, solvent extraction, and adsorption [1,4,5] . Of these, chemical precipitation is the usual way for this purpose . Many factors affect the process of chemical precipitation including the type of precipitation agent, pH, velocity of precipitation, sludge volume, time of mixing and complexing agents [1,6,9] .
The purpose of this research was to compare pH, velocity of precipitation, sludge volume, time of mixing and complexing agent when using the three precipitation agents (lime, sodium hydroxide and magnesium oxide) in the precipitation process of the tannery waste water chromium.

MATERIALS AND METHODS
The chemicals used in this research were raw wastewater from tanning process; solutions of lime, sodium hydroxide 15% and magnesium oxide 10%. All chemicals used are of analytical reagent grade.
Since the process of tanning is batch, after finishing the process (12 h), spot samples were collected and transported to the laboratory for the determination of the most important parameters as quick as possible. These parameters include chromium concentration, pH, total solids, dissolved solids, suspended solids, color, sulfate and chlorine concentration [10] .
The jar test method has been used to determine the effect of each parameter, (six baker has been used for each stage) and 500 ml of wastewater was added to each beaker. Precipitation agents were added to each sample separately and pH was controlled between 6 to 12 using nitric acid. In order to mixed solution, a sample was taken to the jar machine and samples were mixed for one minute in 90 RPM as the first step. In the next step samples were mixed for 20 min with the speed of 30 RPM. After this 20 min. Samples were taken out from the jar machine. In the last stage, after 4 h settling time, a sample was taken from the supernatant. Supernatant solution was then filtered and chromium concentration was determined. The effect of each factor on the three precipitation agents was measured by fixing the effect of the other variables. For instance settling rate, sludge volume and the shape of the precipitation for all precipitation agents were measured when pH was fixed at the optimum level. In order to determine the settling rate and sludge volumes, the samples were poured into scaled cylinders and after 5 h the height of supernatant or sludge volume was read. Atomic Absorption Spectroscopy method (Varian 975) was used for measuring chromium concentration in wastewater before and after settling process [10] .

RESULTS AND DISCUSSION
The chracacterstics of the wastewaters are shown in Table 1. The average of chromium concentration in the raw wastewater of tannery process was 3950 ppm. The results of this study showed that the wastewater of tannery process is one the most important sources of environmental pollutants as the concentration of chromium and other harmful material in the wastewater is extremely high. This is also confirmed by many other studies [11,12] .
The optimum pH for removing chromium from the tannery wastewater by sodium hydroxide and calcium hydroxide is shown in Fig. 1. Figure 1 shows the minimum solution of chromium is presented at pH 8.5 when using sodium hydroxide as the precipitating agent. In case of using calcium hydroxide as the precipitating agent, chromium concentration of settled wastewater decreased as pH increased. Increasing pH resulted in decreasing the chromium concentration in the supernatant. Although the optimum pH for NaOH was 8.5-9.5, the results showed when using CA (OH) 2 as precipitation agent, increasing pH decreased the chromium concentration. This discrepancy can be due to the difference between the ability of the two precipitating agents for dissolving in water. This ability for NaOH is 100%. Resulting chromium hydroxide from using NaOH has the most stability at the pH of 8.5-9.5. However adding more NaOH increases pH and this results in peptizing. In this situation, the chromium redissolve and therefore, the concentration of chromium in supernatant increases. In contrast, since the solubility of CA (OH) 2 is low, once a CA (OH) 2 is added to the wastewater pH increases and increasing pH results in peptizing. However, adsorption causes chromium ions take apart from the supernatant. Panswad et al. Reported that the optimum pH for MgO is 7-8 [13] . The settling rate for the three precipitating agents is shown in Fig. 2. The settling rate of the process for magnesium oxide is much more than this rate for the other two agents. Furthermore, a grainy, dense, easly setteable precipitate can also be formed when MgO is used as the precipitate. Whereas, precipitate forming from NaOH was very gelatinous and the settling rate was low. Sludges forming from CA (OH) 2 was not also desirable. The results of the present study showed that high quality sludge, high settling rate and low volume of sludge was obtained by MgO. Whereas, sludge forming from NaOH and Ca(OH) 2 are very gelatinous. Furthermore, the settling rate for NaOH and CA (OH) 2 is low and dewatering sludge is difficult. Similar to the results obtained by Panswad et al. and Hemming et al. [13,14] , our results showed that MgO is a suitable precipitating agent for removing and recovering chromium from tanning wastewater. Figure 3 shows the sludge volume produced by the three precipitation agents. Sludge volume by MgO is much less than the sludge volume produced by CA (OH) 2 and NaOH. The ratio of sludge is: V MgO : V Ca(OH)2 : V NaOH : 1: 2.5: 3.3 As Fig. 3 shows the sludge volume of chromium hydroxide resulted from the MgO is also much smaller than this volume when using NaOH and CA (OH) 2 . This is important because not only helps to reduce the volume of the treatment plants, but also recovering chromium from such sludges is much easier than sludges with huge volume. The results of Panswad et al. confirm these results [13] .
An optimum time of rapid mixing for the three precipitation agents is shown in Fig. 4. The time needed for rapid mixing of MgO was longer than the other two precipitation agents.

CONCLUSION
It is concluded from our results that the optimum pH for the precipitating chromium from tannery wastewater is 8-9 and the good sludge with high settling rate and lower volume was obtained when using MgO as the precipitating agent. Hence the MgO is a good precipitating agent for removal and recovery of chromium from tanning wastewater. It is also concluded that magnesium oxide is much more desirable than lime and sodium hydroxide for removing and recovering chromium from tannery wastewater.