## 1 Introduction  
With more demands for modern industrial products, heavy-metals consumption is widely increased in many fields. Their environmental impacts bring up serious concern because of their toxicity and non-degradability. Exposure to these pollutants can be harmful to human and environmental species [1; 2; 3; 4]. Lead is one of the toxic metals and is discharged from different industrial activities such as mining, metals production, battery industry, and fuels combustion [5; 6; 7]. Lead's existence in water recourses can cause different diseases such as hepatitis and anemia [8; 9; 10; 11]. According to the World Health Organization (WHO) guideline 2017, the maximum acceptable concentration for lead in drinking water is 0.01 mg/L [12; 13; 14]. Therefore, lead removal from wastewaters has become an important task, which also has the benefit of lead metal recovery. Several remediation techniques have been used for lead recovery/removal from wastewater, such as chemical precipitation, electrodialysis, electrolysis, reverse osmosis, adsorption, and ion exchange [15; 16; 17; 18; 19]. Among these technologies, resin adsorption is one of the most suitable methods for the removal of lead and other toxic metals from aqueous solution due to its low cost, process simplicity, low remnant metal concentration, and high efficiency. Various resins with different functional groups such as strong acid cation (SAC) resins with sulfonic acid group (-SO\({}_{3}\)H), including Amberlite IR-120, Dowex 50W, Purolite C100, Lewatit SP-112, D-001, 732-CR, Dowex 50 WX8, Amberjet 1200 Na, and C 160; weak acid cation (WAC) resins with carboxylic acid group (-COOH) such as Amberlite IRC-50/IRC-86, Lewatit CNP 80, and Purolite C-104; weak acid and chelating resins with iminodiaacetic acid groups such as Purolite S-930, Amberlite IRC 718, and Lewatit TP 207; and acid and chelating resins with amino-phosphonic groups such as Purolite S-940 and Purolite S-950 were studied for lead removal from aqueous solutions [20; 21; 22; 23]. [PERSON] et al. [24] studied the kinetics and thermodynamics of lead removal from aqueous solution using 732 cation-exchange resin with sulfonic acid group in hydrogen type in the temperature range of 298-328 K and Pb(II) concentration range of 5-50 mol/m\({}^{3}\). The experimental results show that the adsorption isotherm data agreed closely with the Langmuir isotherm and the maximum monolayer exchange for Pb(II) was 484.0 mg/g at 308 K. Thermodynamic studies demonstrate that the adsorption of Pb(II) onto 732-CR is spontaneous and exothermic in nature. This resin was also tested by [PERSON] et al. for removing lead from aqueous solutions [25]. The maximum sorption capacity of Pb(II) at pH 4.0 was 396.8 mg/g resin from their study. [PERSON] et al. [26] investigated removal of lead from aqueous system using Amberlite IR-120 with sulfonic acid group and compared its efficiency with natural zeolite. [PERSON] et al. [27] investigated the performance of WAC resin Lewatit CNP80 with carboxylic (-COOH) functional group for Pb(II) removal from battery industry wastewater. The Pb(II) removal efficiency in batch experiments was found to be 83.3% at 25 \({}^{\circ}\)C, with resin dosage of 2 g/L in 6 h contact time. The Pb(II) removal efficiency could increase slightly from 83% to 87% with increase in temperature. A similar study was reported by [PERSON] and [PERSON] using resin (110-H) with carboxylic group for Pb(II) adsorption in HAc-NaAc medium, obtaining the maximum lead adsorption capacity of 485 mg/g at pH 6.7 and 298 K [28]. [PERSON] et al. [29] synthesized thiol-functionalized mesoporous silica by co-condensation of tetraethoxysilane and varying contents of 3-mercaptoethanolpropyltrimethoxysilane in acidic medium with the block copolymer Pluronic 123 as a structure directing agent. Adsorption of lead from water by the thiol-functionalized silica was studied.