Water pollution is one of the most important environmental problems in human societies. It mainly arises from wastewater released from household, industrial, and agricultural processes. These effluents typically contain high concentrations of organic and inorganic chemicals such as hydrocarbon solvents, heavy metals, dyes, pesticides, and so on [1]. Azo dyes, one of the greatest groups of synthetic dyes, are widely used in many industries. Therefore, removal of azo dyes from colored effluents of industeries has attracted great interest due to their complex composition, toxicity, poor degradability, and high solubility [2]. Zero-valent iron (ZVI) nanoparticles (NPs) have been used as reactive medium for wastewater treatment because the iron metal is low-cost, easy-to-obtain, and environmentally friendly [3,4]. However, nano ZVI shows some shortcomings such as low reactivity and incomplete degradation due to the formation of oxide layers which block the surface active sites [4]. An appropriate technique to overcome this limitation is combining a second metal to ZVI to design a bimetallic system which can benefit from the features of both monometals simultaneously. Our goals of the present work are to prepare Fe/Pd bimetallic NPs, investigate the ability of these NPs for an azo dye removal, and study the kinetics and thermodynamics of the process. Methods Fe/Pd NPs were synthesized by successive reduction method. FeSO4.7H2O and palladium acetate were used as the precursors of Fe and Pd and NaBH4 as the reduction agent. Results and discussion In this work, successive reduction method was used for deposition of a trace amount of Pd on nano Fe surface to produce engineered Fe/Pd bimetallic NPs. Ten characterization methods such as X-ray diffraction, X-ray fluorescence spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy (TEM), high resolution TEM, and BET surface area analysis were used to clarify the formation of Fe/Pd bimetallic NPs and to confirm the presence of Pd on the surface of Fe NPs. The ability of Fe/Pd NPs for removal of an azo dye, Reactive Black 5 (RB5), was investigated. RB5 was selected as a pollutant model due to its wide usage in textile industry. The influence of Fe/Pd dosage, pH, temperature, and initial dye concentration on the dye removal process was studied. The synergetic effect of Pd on Fe leads to an efficient and fast dye removal process, i.e, the maximum dye removal (100%) was achieved for 20.0 mg Fe/Pd, pH=3.0, and the dye concentration of 20 ppm at 25 o C in less than 15 min. Kinetic studies showed that RB5 removal using Fe/Pd bimetallic NPs obeys the pseudo-first order kinetic model. Thermodynamic study of RB5 removal revealed that the process was spontaneous (ΔG= -8.32 kJ mol -1 ) and exothermic (ΔH= -4.06 kJ mol -1 ). The HPLC-MS technique was used to propose a possible mechanism for RB5 degradation. The results confirm that dye molecules are reduced to smaller aromatic compounds via cleavage of azo bonds without producing more hazardous materials. Conclusion The ability of iron NPs for an efficient degradation of RB5 dye was improved by deposition of a trace amount of Pd NPs on Fe surface. The results showed that the acidic pH, increasing dosage of NPs, and low temperature favour the removal of RB5. Also, the mechanism of the RB5 removal using Fe/Pd was discussed and the evidences confirmed that the RB5 dye molecules were reduced to some smaller aromatic compounds via cleavage of azo bonds. References [1] G. Crini, Bioresour. Technol. 97 (2006) 1061-1085. [2] E. Forgacs, T. Cserháti, G. Oros, Environ. Int. 30 (2004) 953-971. [3] W.J. Epolito, H. Yang, L.A. Bottomley, S.G. Pavlostathis, J. Hazard. Mater. 160 (2008) 594-600. [4] S. Chatterjee, S.-R. Lim, S.H. Woo, Chem. Eng. J. 160 (2010) 27-32.

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