Based on the XRD illustration of pumice and zeolite shown in Figure 1a and b the major constituents of pumice include hematite, quartz, ilit and mica, while the major constituents of zeolite are cilnoptilolite, quartz, mordenite and calcite. Table 1 showed that there are two major elements including SiO2 and Al2O3 for pumice with 51.45% and 17.08%, and for zeolite with 63.12% and 12.60%.
The surface area is an effective factor in arsenic adsorption capacity by the adsorbent .The SEM images of ACP and ACZ in Figure 1c and d showed ordered silica crystals and micro small cracks found on ACP surfaces. Also, ACZ had significant rougher surfaces than ACP. Results of the measurement of surface area can be matched with Figure 1, because the BET surface area of the ACP was measured as 7 m2/g, while the BET surface area of the ACZ was measured as 15 m2/g.
Figure 2 shows more than 95%, arsenic uptake by ACP and ACZ with initial As(V) concentration of 2000 μg/L. Also in this concentration of As(V), removal by both adsorbents have been increased rapidly from 2.5 to 20 g/L, and marginally thereafter.
When the adsorbent dose increases from 2.5 to 20 g/L, there is more specific surface for arsenate adsorption, but for higher adsorbent dosage, the removal efficiency does not increase. Also, the amount of As (V) adsorption increases when the initial concentration decreases and the adsorbent dose remains constant, because there is more specific surface for As(V) adsorption. Figure 2 also showed that ACZ was more effective than ACP for arsenic removal, because more than 96% of As (V) with initial concentration of 250 μg/L was removed by ACZ, while only 71% of As (V) could be removed by ACP with the same concentration. The reason is the larger specific surface area of ACZ in comparison with ACP. In most of the previous studies, such as arsenic adsorption onto iron oxide and aluminum  and natural laterite  as well as activated alumina impregnated with alum , the arsenic removal was shown to be increased by increasing the adsorbent dose and decreasing arsenic initial concentration.
The adsorption of As (V) by both adsorbents were very high at pH range of 4 to 8 and the maximum level for ACP and ACZ were at pH = 7 with the amount of 71.2% and 95.8%, respectively. Thereafter, the amount of adsorption decreased remarkably at higher pH values; only 35% and 66.1% of the arsenic adsorption by ACP and ACZ occurred at pH = 11, respectively. It is to be mentioned that the same process has been seen in the previous studies such as arsenic adsorption to iron-modified high expanded clay aggregates , arsenic removal by pretreated waste tea fungal biomass , arsenate adsorption onto iron and aluminum oxides , arsenic adsorption onto rare earth oxides  and arsenic adsorption onto activated aluminum impregnated with alum . When pH increases, the adsorption decreases because the adsorption surface is negatively charged and columbic repulsions increase .
In the pH range of 3–11, arsenate is predominantly presented in the species of H2
. Therefore, it can be concluded that those are the major species being adsorbed on the surface of ACP and ACZ. The adsorption process of arsenate by both adsorbents is as equations 10
As it is evident one of the benefits of these adsorbents is their good performance in pH value of natural waters and it is not required to use acid or alkali for adjusting pH.
In order to estimate the rate of adsorption and determine the behavior of the adsorptive, the adsorption kinetics of As(V) onto ACP and ACZ were studied at different intervals of time. Based on the results of the investigations, as shown in Table 2, during the first 5 min, 40% and 88.4% of the arsenic adsorption onto ACP and ACZ were obtained rapidly, while 58.2% and 11.4% of the adsorption for both adsorbents occurred during the next 155 min, respectively. Maximum adsorption of 95.2% and 98.8% (for ACP and ACZ, respectively) were observed at 160 min within the equilibration time.
The rate constants of As(V) adsorption were calculated by the rate expression of pseudo-first order and pseudo-second order models which have been previously described. In pseudo-first order model, the constancy rate of adsorption (K1) and the correlation coefficient (R2) for ACP and ACZ were found to be 0.0010, 0.0009, and 0.80,0.85, respectively (Figure 4a); low R2 shows that the adsorption of As(V) onto ACP and ACZ does not follow pseudo–first order model. In pseudo-second order model the constancy rate of adsorption (K2) and R2 for ACP and ACZ were found to be 0.0128, 0.0136, and 0.98, 0.99, respectively (Table 3). The low K2 and high R2 values suggest that the adsorption for both adsorbents to be under control of pseudo-second order model (Figure 4b). Similar rapid arsenic adsorption has been reported such as As(V) uptake by pretreated waste tea fungal biomass in which adsorption was relatively fast at the initial As(V) concentration of 4 mg/L. In this process adsorption reached the equilibrium within 90 min and the pseudo second-order model described the biosorption kinetics of As(V) with good correlation coefficient (R2 > 0.93) . The biosorption of As(V) by P. chrysogenum reached over 70% of equilibrium uptake capacity in 10 min . Arsenic removal by nanocrystalline TiO2 occurred rapidly and system reached equilibrium in 4 h; results were best described by pseudo-second order model (R2 > 0.93) .
In order to design an appropriate sorption system to remove As(V) from drinking water, it is important to find the well-fitted isotherm curves of ACP and ACZ. As it is evident from the R2 values in Table 4, the Freundlich isotherm yielded better fitting to the experimental data for both adsorbents, probably due to the heterogeneous natures of their surface sites involved in the arsenic uptake. This result also signifies that surface energy does not remain constant during the process of adsorption by ACP and ACZ, but varies with the surface coverage . A similar trend has been observed in the case of phenol and 4-chlorophenol adsorptions on to pumice treated with cationic surfactant  and in arsenic removal from simulated industrial wastewater by magnetite nanoparticles . The Kf and n values in Freundlich isotherm model for As(V) adsorption onto ACP and ACZ at 20°C were found to be 0.07, 0.54 and 0.86, 0.62, respectively.
The quality of water after treatment regarding the aluminum concentration was analyzed. Results showed that in maximum adsorption of As(V) at pH = 7 and 160 min by 10 g/L ACP and ACZ, 95.2% and 98.8% of arsenic was removed and reached below 12 and 3 μg/L, respectively. After treatment the soluble aluminum concentration for ACP and ACZ were 0.01 and 0.03 mg/L respectively, which is below the permissible limit set by the Institute of Standard and Industrial Research of Iran (0.2 mg/L).
Finally ACZ due to having higher porosity and specific surface area may be considered as an efficient adsorbent for providing higher adsorption capacity.