Synthesis of walnut shell modiﬁed with titanium dioxide and zinc oxide nanoparticles for efﬁcient removal of humic acid from aqueous solutions Ali Naghizadeh, Habibeh Shahabi, Fatemeh Ghasemi and Ahmad Zarei ABSTRACT The main aim of this research was to study the efﬁciency of modiﬁed walnut shell with titanium dioxide (TiO 2 ) and zinc oxide (ZnO) in the adsorption of humic acid from aqueous solutions. This experimental study was carried out in a batch condition to determine the effects of factors such as contact time, pH, humic acid concentration, dose of adsorbents (raw walnut shell, modiﬁed walnut shell with TiO 2 and ZnO) on the removal efﬁciency of humic acid. pH zpc of raw walnut shell, walnut shell modiﬁed with TiO 2 and walnut shell modiﬁed with ZnO were 7.6, 7.5, and 8, respectively. The maximum adsorption capacity of humic acid at concentration of 30 mg/L, contact time of 30 min at pH ¼ 3 in an adsorbent dose of 0.02 g of walnut shell and ZnO and TiO 2 modiﬁed walnut shell were found to be 35.2, 37.9, and 40.2 mg/g, respectively. The results showed that the studied adsorbents tended to ﬁt with the Langmuir model. Walnut shell, due to its availability, cost-effectiveness, and also its high adsorption efﬁciency, can be proposed as a promising natural adsorbent in the removal of humic acid from aqueous solutions. Ali Naghizadeh (corresponding author) Habibeh Shahabi Fatemeh Ghasemi Department of Environmental Health Engineering, Faculty of Health, Birjand University of Medical Science (BUMS), Birjand, Iran E-mail: aliinaghizadeh@gmail.com Ali Naghizadeh Medical Toxicology and Drug Abuse Research Center (MTDRC), Birjand University of Medical Sciences (BUMS), Birjand, Iran Ahmad Zarei Department of Environmental Health Engineering, Faculty of Health, Gonabad University of Medical Science, Gonabad, Iran Key words | adsorption, humic acid, titanium dioxide, walnut shell, zinc oxide INTRODUCTION Humic and fulvic acids are known as the main precursors of trihalomethanes and other disinfection by-products in water (Graham ). Humic acid is a major component of organic compounds originating from decomposition of dead plant materials, animal bodies, and biological activities of microor- ganisms ( Jiahong et al. ) which exists in a yellow to black color in almost all natural waters (Corin et al. ). Humic acid may cause water to have an undesirable taste and color. Moreover, it acts as a nutrient for the growth of bacteria in water distribution systems. Humic acid can cause blockage of membranes in membrane treatment processes, and there- fore, increase the operational costs. Furthermore, humic acid can form complexes with chlorine, heavy metals, pesticides, and herbicides, creating carcinogenic compounds (Graham ; Rubia et al. ; Naghizadeh ). Thus, even low levels of humic acid can cause major problems in water. Various processes, such as chemical coagulation, advanced oxidation, membrane separation, adsorption and bio-degra- dation, have been studied for the removal of humic acid from water. Among these, adsorption is most used due to its simpli- city and cost-effectiveness. Due to its characteristics, many studies have been carried out in order to develop a suitable adsorbent for the removal of humic acid from water (Duan et al. ; Lorenc-Grabowska & Gryglewicz ; Salman et al. ; Wang et al. , ). Recently, a great deal of atten- tion has been paid to the adsorption process. Adsorption efﬁciency depends on several operating variables, such as sol- ution pH, adsorbent dosage, contact time, type of adsorbent and organics (Dehghani et al. ; Naghizadeh ). In the case of expensive adsorbent application, the adsorption pro- cess will also be expensive. Therefore, it is better to use natural adsorbents such as agricultural wastes which are 989 © IWA Publishing 2016 Journal of Water and Health | 14.6 | 2016 doi: 10.2166/wh.2016.072 Downloaded from http://iwaponline.com/jwh/article-pdf/14/6/989/394162/jwh0140989.pdf by guest