RESEARCH ARTICLE Photocatalytic degradation of humic substances in aqueous solution using Cu-doped ZnO nanoparticles under natural sunlight irradiation Afshin Maleki 1 & Mahdi Safari 1 & Behzad Shahmoradi 1 & Yahya Zandsalimi 1 & Hiua Daraei 1 & Fardin Gharibi 2 Received: 15 February 2015 /Accepted: 16 June 2015 # Springer-Verlag Berlin Heidelberg 2015 Abstract In this study, Cu-doped ZnO nanoparticles were investigated as an efficient synthesized catalyst for photodegradation of humic substances in aqueous solution under natural sunlight irradiation. Cu-doped ZnO nanocatalyst was prepared through mild hydrothermal method and was characterized using FT-IR, powder XRD and SEM tech- niques. The effect of operating parameters such as doping ratio, initial pH, catalyst dosage, initial concentrations of hu- mic substances and sunlight illuminance were studied on hu- mic substances degradation efficiency. The results of charac- terization analyses of samples confirmed the proper synthesis of Cu-doped ZnO nanocatalyst. The experimental results in- dicated the highest degradation efficiency of HS (99.2 %) ob- served using 1.5 % Cu-doped ZnO nanoparticles at reaction time of 120 min. Photocatalytic degradation efficiency of HS in a neutral and acidic pH was much higher than that at alka- line pH. Photocatalytic degradation of HS was enhanced with increasing the catalyst dosage and sunlight illuminance, while increasing the initial HS concentration led to decrease in the degradation efficiency of HS. Conclusively, Cu-doped ZnO nanoparticles can be used as a promising and efficient catalyst for degradation of HS under natural sunlight irradiation. Keyword Humic substances . Photocatalytic . Sunlight irradiation . Nanoparticles . Degradation . Cu-doped ZnO Introduction Humic substances (HS) are the major fraction of natural or- ganic matters (NOM) originated from microbial degradation of plant and animal tissues (Patsios et al. 2013; Ulu et al. 2014; Turkay et al. 2015). The presence of HS in drinking water cause undesirable effects on water quality and human health. HS may lead to colour, taste and odour problems in drinking water (Mahvi et al. 2010; Nkambule et al. 2012; Patsios et al. 2013). Transport of contaminants, microbial regrowth in the distribution system and membrane fouling problems are asso- ciated with the presence of HS in water resources (Valencia et al. 2013; Ulu et al. 2014). In addition, HS are major precur- sors of disinfection by-products (DBPs) such as trihalometh- anes (THMs) and haloacetic acids (HAAs), which are muta- genic and carcinogenic (Mahvi et al. 2010; Turkay et al. 2015; Valencia et al. 2013; Xue et al. 2011). Many treatment process such as electrocoagulation (EC) (Ulu et al. 2014), adsorption (Li et al. 2011), nanofiltration (Rajesh et al. 2013), catalytic ozonation (Choi and Rhee 2014), enhanced coagulation (Amin et al. 2012) and photocatalysis (Hatakeyama et al. 2012; Yuan et al. 2013) have been developed for elimination of HS from water. In the past decade, photocatalysis processes have emerged as an efficient technology for removing of organic compounds from aquatic solutions (Gaya and Abdullah 2008; Patsios et al. 2013). Semiconductor metal oxides such as ZnO and TiO 2 , as catalyst, have attracted much attention for photocatalytic deg- radation of pollutants (Huang et al. 2008; Nkambule et al. 2012; Liu et al. 2014; Saleh and Djaja 2014). Although TiO 2 catalyst has been widely used for photocatalytic processes in recent years, ZnO catalyst is a suitable alternative due to its crucial properties such as wide band gap energy, high produc- tion of H 2 O 2 and low cost and high potential to adsorb UV irradiation (Ullah and Dutta 2008; Darvishi Cheshmeh Soltani Responsible editor: Philippe Garrigues * Mahdi Safari Safari.m.eng@gmail.com 1 Environmental Health Research Center, Kurdistan University of Medical Sciences, Sanandaj, Iran 2 Research Deputy, Kurdistan University of Medical Sciences, Sanandaj, Iran Environ Sci Pollut Res DOI 10.1007/s11356-015-4915-7