## 2 Results and Discussion
### Adsorption  
#### 2.5.1 Effect of pH on Cr(VI) Adsorption and Sorption Mechanism  
The pH of the Cr(VI) solution plays an important role in the adsorption process. It has an effect on the conversion of chromium species in the solution as well as the surface charge and the protonation degree of functional groups on the active sites of the adsorbent. There are five main forms of Cr(VI) in the aqueous solution, including Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\), HCr\({}_{2}\)O\({}_{7}\), CrO\({}_{4}\)\({}^{2-}\), HCrO\({}_{4}\)\({}^{-}\) and H\({}_{2}\)Cr\({}_{2}\)O\({}_{4}\)n [36]. The stability of these forms is dependent on the pH of the system. When pH < 1, the chromium ions exist in the form of H\({}_{2}\)CrO\({}_{4}\), while HCrO\({}_{4}\)\({}^{-}\) and Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\) are predominant at pH 2.0-6.0 and CrO\({}_{4}\)\({}^{2-}\) is the dominant component of Cr(VI) above pH 6.0 [32]. In this study, the effect of initial pH value on Cr(VI) adsorption amount on EDA-MPMs was illustrated in Figure 5 at different pH values ranging from 1.0 to 8.0. The results showed that the maximum adsorption capacity on EDA-MPMs was at pH 2.0. At pH 2.0, the amino groups of the adsorbent was the protonated cations (-NH\({}_{3}\)\({}^{+}\)) which resulted in a stronger attraction for a negatively charged ion in the solution. The negatively charged chromium species (HCrO\({}_{4}\)\({}^{-}\) and Cr\({}_{2}\)O\({}_{4}\)\({}^{2-}\)) were the predominant species, which were easily attracted through electrostatic attraction to the positively charged surface of the EDA-MPMs. Therefore, the maximum adsorption capacity was achieved at pH 2.0 (148 mg/g) in the initial Cr(VI) concentration of 300 mg/L, which was consistent with the previous study [28; 37]. But in acidic medium (pH < 2), the neutral H\({}_{2}\)CrO\({}_{4}\) was the main component, which brought about the decline of electrostatic attraction between adsorbent and Cr(VI). Hence, the adsorption capacity decreased as the pH decreased from pH 2.0 to 1.0 due to the involvement of less number of HCrO\({}_{4}\)\({}^{-}\). At higher pH, the adsorption capacity decreased due to the competition of both the anions (CrO\({}_{4}\)\({}^{2-}\) and OH\({}^{-}\)) to be adsorbed on the surface of the adsorbent of which OH\({}^{-}\) tend to repulse the metal anions (CrO\({}_{4}\)\({}^{2-}\)). Therefore, the optimal pH value of 2.0 was selected for the subsequent experiments.  
Figure 4: FT-IR spectra of MPMs (**a**) and EDA-MPMs (**b**).  
Figure 5: Effect of the pH on the Cr(VI) adsorption onto EDA-MPMs.  
The adsorption process of the Cr(VI) can be explained in terms of the electrostatic interactions between the positively charged protonated functional groups (-NH\({}_{3}\)+) on EDA-MPMs surface and the negative charge chromium species (HCrO\({}_{4}\)- and Cr\({}_{2}\)O\({}_{4}\)2-). The amino groups of EDA-MPMs were protonated under acidic conditions according to the following reaction.  
\[\text{R-NH}_{2}\,+\,\text{H}^{+}\leftrightarrow\text{R-NH}_{3^{+}} \tag{1}\]  
The chromium adsorption reactions onto EDA-MPMs are shown in Equations (2) and (3).  
\[\text{HCrO}_{4^{-}}\,+\,\text{R-NH}_{3^{+}}\leftrightarrow\text{HCrO}_{4^{-}} \cdots\,^{+}\text{NH}_{3}\text{-R} \tag{2}\]  
\[\text{Cr}_{2}\text{O}_{4^{2}}\,+\,\text{R-NH}_{3^{+}}\leftrightarrow\text{ CrO}_{4^{2}}\cdots\,^{+}\text{NH}_{3}\text{-R} \tag{3}\]