Removal of zinc and cadmium heavy metals from industrial wastewater using organic and nano oxide adsorbents

Nowadays, due to the reduction of water resources, the use of unconventional water sources such as wastewater has been given special attention in the world. The presence of some heavy metals in wastewater particularly in industrial wastewater limits the application. This study was carried out in order to investigate the efficiency of some organic and nano oxide adsorbents in the removal of zinc (Zn) and cadmium (Cd) from industrial wastewater.

The study was done at four steps including the effects of 1), adsorbent mass, 2) exposure time, 3) concentration of heavy metals, and 4) pH on the removal of Zn and Cd from wastewater. At first, an artificial wastewater was created based on 40 mg l-1 Zn and 25 mg l-1 Cd. Four adsorbents including wheat straw and palm fiber as organic adsorbents and titanium nano oxide and iron nano oxide as nano adsorbents each at five doses (0.2, 0.5, 1, 1.5 and 2 g l-1) were applied. Different time durations were implemented in order to assess time effect in addition to the equilibrium time. In addition, the influence of pH in a range of 3 to 6 on the efficiency of adsorbents was examined. Moreover, in order to study the effect of different concentrations of Zn and Cd on the efficiency of adsorbents, three levels of pollution concentrations were generated by the combinations of 40, 80 and 120 mg l-1 Zn and 25, 50 and 75 mg l-1 Cd.

The result showed that the adsorption of heavy metals increased with increasing the dose of each adsorbent in the wastewater. As the application rate of palm fiber, wheat straw, titanium nano oxide and iron nano oxide increased from 0.2 to 2 g l-1, the adsorption of Zn increased by 49.4%, 17.1%, 13.1% and 11.4 %, and the adsorption of Cd increased by 41.1%, 3.8%, 18.0% and 1.5%, respectively. Overall, palm fiber with the optimal level of 1.5 g l-1 exhibited the best efficiency in the removal of Zn (87.2%) and Cd (46.2%). The equilibrium time for the adsorption of both Zn and Cd using wheat straw, palm fiber, titanium nano oxide and iron nano oxide was 300, 120, 30 and 30 minutes, respectively. In fact, the nano adsorbents reached the equilibrium conditions faster than the organic adsorbents. The applied organic adsorbents at pH of 6 and the nano oxide adsorbents at pH 4 had the highest efficiency in Zn adsorption. In the case of Cd adsorption, wheat straw at pH 5, date fibers at pH values of 6 and 3, iron nano oxide at pH values of 4 and 5, and titanium nano oxide at pH values of 4 and 3 showed the highest efficiency. The organic adsorbents at lower levels of pollution and the nano adsorbents at higher levels of pollution found to be more effective to remove the heavy metals from the wastewater.

The findings of this study revealed that during the removal process of Zn and Cd from wastewater, the organic adsorbents had higher adsorption, whereas the nano adsorbents reached the equilibrium conditions sooner.