Adsorption mechanism of used azo dyes can be easily described from the examination of the chemical composition of adsorbent and dyes. As shown in Table 1
, the main component of the sorbent was SiO2
(51.45%). According to the following equation, in aqueous solution this component can discompose to Si+2
at the surface of the adsorbent, leading to the formation of Si+2
species at the surface of the adsorbent:
In addition, in aqueous solution acid dyes are first dissolved and then are discomposed to the sulfonate groups of acid dye and hence to the anionic dye group as follows:
In light of Eq. ((16)) and Eq. ((17)), the adsorption process continued due to electro-static interaction of DSO3-
as two counter groups:
Moreover, in the presence of protons, they are added to the surface of the adsorbent increasing the cationic capacity of the adsorbent [15
From the above equation, the sorption capacity of pumice in acidic condition would increase. Differences in removal yields between dyes can be ascribed to the molecular weight of dyes, their chemical structure and the number of sulfonate groups. AR18 has a high molecular weight in comparison with AR14. The lower molecular weight of AR14 allowed a more efficient penetration in the internal part of the adsorbent, leading to a higher removal yield than AR18. In addition, a lower number of sulfonate groups as was the case for AR 14 if compared to AR 18 resulted in a lower size and a lower strict hindrance and hence the penetration in the adsorbent was easier. These phenomena can be considered to account for the results obtained from equilibrium study and desorption experiment as seen below.
As shown on Figure 1 and Table 2, the pumice adsorbent showed an irregular structure after modification. The irregular structure is responsible for increasing in dye sorption capacity. In addition, pumice stone shows smaller pore structure after treatment by HCL. The initial specific surface of pumice stone was 28 m2/g and increased to 54 m2/g after acidic treatment.
AR14 has a low molecular weight and hence can penetrate into the internal part of pumice adsorbent, in agreement with previous results recorded using fly ash as adsorbent . In addition, previous results recorded using activated carbon prepared from Poplar wood , which displayed a high specific surface area (385m2/g) but a low porosity, showed lower yield and rate of removal of AR18, if compared to the present results obtained on pumice stone. It should be most likely related to the high porosity of pumice stone, showing adsorption not only at the surface, but also in the internal part of the sorbent.
Removal yields increased almost linearly for increasing initial dye concentrations (Figure 3), which can most likely be attributed to increasing driving force for increasing initial dye concentrations . Irrespective of the initial dye concentration, higher removal yields were recorded for AR14, if compared to AR18. This phenomenon can be described by large number of mere sites for adsorption of dye at initial stage and with time elapse, and owing to the repulsive forces between sorbate and bulk phase, the occupation of the remaining sites became more difficult to dye molecules . In addition and based on isotherm study described thereafter, due to the higher affinity of AR14 if compared to AR18, higher removal rates were observed for the former at initial sorption stage.
The pH has an important role in the adsorption process. According to the pH, the surface of the adsorbent will be positively or negatively charged. As the pHzpc
of pumice was 7.2, at higher and lower values the surface of pumice will be occupied by OH-
ions, respectively. When introduced into the solution, Azo dyes dissolved and formed negative sulfonate groups. Due to the positive charge of pumice in acidic conditions, the following reaction would occur:
Therefore, acidic conditions leading to increasing amount of H+ ions, especially at the surface of pumice, consequently improve dye removal [15, 16].
A low temperature was therefore more suitable for AR14 and AR18 removal, showing that the adsorption was exothermic in nature. An escape of dye molecules from the solid surface to the solution for an increment in temperature can account for this behavior.
Fitting of obtained data onto three isotherm model show the removal of AR 14 and AR 18 follow Freundlich (r2>0.99) and Langmuir (r2>0.99) isotherm model. Maximum sorption capacities were 29.7 and 3.125 mg/g for AR 18 and AR 18, respectively; according to Langmuir isotherm constant. The high value of Kf was observed for AR 14 rather than AR 18, showing high affinity of AR 14 onto pumice stone. The value of Freundlich constant (n) was 5.4 and 1.4 for AR 18 and AR 14, respectively; showing the favorable nature of present sorption process. Removal of AR14 by three soils, namely GSE17200, GSE1201 and DG06 has been studied by Baocheng et al. , and showed that AR14 removal followed a Freundlich isotherm model. Maximum capacities of DG06, GSE17200 and GSE17201 were 1.3, 0.98 and 0.83 (mg/g) respectively, namely below that of pumice for AR14 (3.125 mg/g). AR18 removal was previously investigated using activated carbon derived from Poplar wood . The Kf value and maximum sorption capacity were 1.5 and 3.9 (mg/g) respectively, namely significantly lower than those obtained in this work using pumice stone, 12.17 and 29.7 (mg/g) respectively.
It is clear from Table 4 that removal of AR18 and AR14 followed Intra-particle diffusion (r2>0.98) and pseudo-second order (r2>0.99) kinetic model, respectively. However, regression of intra-particle diffusion model did not pass through the origin, showing that it was not the rate-determining step. It can also be observed, that for both dyes similar values were observed for calculated and experimental qe.
The values of ∆H0 and ∆S0 for AR14 were found to be −2698 (kJ/mol) and −9.56 (kJ/mol.K), respectively; while for AR18 they were −3165.88 (kJ/mol) and −11.35 (kJ/mol.K), respectively. The negative value of enthalpy change (∆H0) for both dyes demonstrated that adsorption of AR14 and AR18 onto the present medium was exothermic in nature. The negative values of ∆S0 for both dyes indicated a decrease in the fortuitousness of adsorption at the solid/liquid interface. On the other hand, Gibbs free energy (∆G0) for AR14 and AR18 were found to be in the ranges 124.9- 494.7 and 191.3-667.4 for temperatures in the range 293–333 K, respectively. The positive values of the Gibbs free energy for both dyes indicated that sorption of AR14 and AR18 onto acidic treated pumice were not thermodynamically spontaneous.
For both dyes the values of external mass transfer coefficient decreased for increasing initial dye concentrations. Increasing values of kmtc can be related to increasing external mass transfer resistance at solid/liquid layer. The result of external mass transfer coefficient for both dyes was in the order of 10-3-10-4, in agreement with values of order of magnitude 10-4 found in the literature [23, 24].
Desorption experiments are needed to investigate the economic feature of adsorption. As shown in Figure 4, dye removal was low at high pH, which demonstrated that desorption can be done at alkaline pH. Heating the used adsorbent is another way to perform desorption. In this work, both methods were investigated. By heating method, about 63% and 76% regeneration was observed for AR14 and AR18, respectively. High regeneration of pumice for AR18 can be ascribed to the low affinity of AR18 (Kf=12.17) onto pumice surface and the release of SO3- from the surface of the adsorbent. By pH method, the adsorbed regeneration percentages were 64%, 72% and 86% for AR14 and were 65%, 71% and 89% for AR18 for 1, 1.5 and 2 N NaOH, respectively. Resulting from pH increase, the surface of the adsorbent bombed by OH- ions led to the extraction of SO3- and H+ from the surface of the adsorbent. In addition, high OH- ion concentrations increased the driving force of others ions to be adsorbed on the surface of the medium.
From the above comments, it can be concluded that regeneration of pumice can be efficiently done by the heating method.