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Optimization and Characterization of Tl(I) Adsorption onto Modified Ulmus carpinifolia Tree LeavesUlmus carpinifolia tree leaves were successfully used to remove Tl(I) from aqueous solution in a batch system. In order to improve the uptake capacity of sorbent, it was modified by various chemical agents such as NaOH, HNO 3 , NH 3 , NaCl, NaHCO 3 , and CaCl 2 . Among the modifiers, NaCl was the best. Equilibrium behavior of sorbent with Tl(I) was examined by the several isotherms. Considering modified U. carpinifolia equilibrium data fitted well to the Langmuir model with maximum capacity of 54.6 mg/g. The other isotherms such as: Freundlich and Dubinin-Redushkevich (D-R) models were also examined. The central composite design (CCD) was successfully employed for optimization of biosorption process. An empirical model was given through using response surface methodology. Also its validation was recognized by using relevant statistical tests such as ANOVA. The optimum conditions of biosorption: pH, m (amount of sorbent) and C (initial concentration) were found to be 7.9, 11.4 g/L, and 8.8 mg/L, respectively. On the other hand thermodynamic parameters: DG, DH, and DS were evaluated: the obtained results show that biosorption process was spontaneous and exothermic. Eventually, FT-IR analysis confirmed that the main functional groups of sorbent have been involved through the biosorption process. IntroductionThe presence of heavy metals in the environment poses a problem due to their harmful affects on human health and biota. Thallium is a heavy metallic element that exists in the environment mainly combined with other elements (primarily oxygen, sulfur, and halogen) [1]. Thallium (Tl) is considered as a non-essential and highly toxic element, which is produced as a by-product in the refining of iron, cadmium, and zinc [2,3]. It is used as a catalyst in certain alloys, optical lenses, jewels, low-temperature thermometers, semiconductors, metal mining, ore processing and combustion of coal [1,2]. Thallium is more toxic to humans than mercury, cadmium, lead, copper, or zinc and has been responsible for many accidental, occupational, deliberate, and therapeutic poisonings since its discovery in 1861 [4]. It occurs almost exclusively in natural waters as monovalent thallous cation. The solubility of thallous compounds is relatively high so that monovalent thallium is readily transported through aqueous routes into the environment. The fascinating chemistry and high toxicity potential make thallium and its compounds of particular scientific interest and environmental concern [4]. Thallium is present mainly in industrial wastewaters [5]. Due to these adverse biological and environmental effects, it is required to have some improved methods for detecting, monitoring, and removing it from environmental sources and industrial waste effluents. The conventional waste water treatments which used for removal of heavy metal ions are including: Chemical precipitation, ion exchange, and membrane technologies, adsorption onto activated ...

A B S T R A C TCatalpa speciosa tree leaves were used to remove Pb(II) from aqueous solutions. Removal process was optimized by applying experimental design strategy. Optimization was based on the application of a two-level full-factorial design for screening the significant variables followed by a Doehlert design to find out an appropriate model which leads to determine the optimum conditions. The empirical models were developed in terms of effective factors: pH, initial concentration of metal ion C m and sorbent mass(s) have been found statistically adequate to describe the process responses, that is, removal percent and the capacity uptake of Pb(II). The multiresponse optimization of Pb(II) removal process has been carried out using desirability function approach. To this end, Derringer's desirability function (D) has been applied for mathematical optimization of the simultaneous multiresponse problem. Thus, the optimum conditions of Pb(II) removal process have been found as: pH = 4.43, s = 0.07 g, and C m = 104 mg L −1 . Responses were confirmed by experiments 86 and 14.7 mg g −1 for R% and q, respectively. The morphology and structure of biosorbent during adsorption process was characterized by FT-IR spectroscopy, X-ray diffraction and scanning electron microscopy techniques. Experimental data were also fitted to various isotherm models. Potentiometric titration, pH PZC , and the competitive effect of alkaline and alkaline earth metal ions during the loading of Pb(II) indicate that ion exchange is the predominant mechanism for the adsorption of Pb(II) by C. speciosa tree leaves.

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