## 3 Results and Discussion
### Effect of pH and Ionic Strength on Sorption  
Aqueous phase pH would govern the adsorption process as it affects the surface charge of the adsorbent, the degree of ionization and the dominant speciation of the metal contaminant [4]. Besides, a change of pH could influence the reaction rate of iron oxidation [15]. Thus, the effect of pH on heavy metals removal by PPG-nZVI beads was conducted over a pH range of 3.0-5.5. For pH above 5.5, adsorption experiments were not conducted because the precipitation for Cu\({}^{2+}\) could occur. As seen from Figure 4a, the removal efficiency of Pb\({}^{2+}\), Cu\({}^{2+}\), and Zn\({}^{2+}\) by PPG-nZVI beads increased with the increase of pH. From pH 3.0-5.5, the removal rate increased from 49.0-92.7%, 21.7-81.9%, and 18.6-48.4% for Pb\({}^{2+}\), Cu\({}^{2+}\), and Zn\({}^{2+}\), respectively. The results indicated that the removal of different heavy metal ions by PPG-nZVI beads was strongly pH-dependent. In Fe\({}^{0}\) treatment systems, the removal mechanisms of Pb\({}^{2+}\), Cu\({}^{2+}\), and Zn\({}^{2+}\) are generally believed to involve the adsorption of the ions on the iron surface where electron transfer takes place. Pb\({}^{2+}\), Cu\({}^{2+}\), or Zn\({}^{2+}\) is reduced to the formation of Pb\({}^{0}\), Cu\({}_{2}\)O, Cu\({}^{0}\), or Zn\({}^{0}\) with the oxidation of Fe\({}^{0}\) to Fe\({}^{3+}\) or Fe\({}^{2+}\) under acidic conditions [16; 17]. Actually, besides the removal mechanisms regarding to the electron transfer between heavy metals and iron surface, the heavy metal might also interact with PVA molecules and guar gum by electrostatic interaction followed by bridging mechanisms. The explanation is similar with [PERSON] and [PERSON] [18]. Divalent cations such as Pb\({}^{2+}\), Cu\({}^{2+}\), and Zn\({}^{2+}\) is expected to have an important impact during the aggregation of negatively charged beads with anionic polyelectrolytes. It is likely that such metal ions will act as binding agent between anionic polyelectrolyte sites and negative particles (i.e., complexing of metal ions with anionic carboxyl groups), despite the anticipated electrostatic repulsion. The interactions may refer to electrostatic interaction, hydrogen bonding, and/or divalent cation binding. PVA is composed of copolymers of vinyl alcohol and vinyl acetate. The high density of hydroxyl groups on its structure can lead to a more cross-linked configuration. The interparticle bridging behavior in the presence of nonionic PVA is essentially due to hydrogen bonding. The observed lower uptake in the acidic medium may be attributed to the partial protonation of the active groups and the competition of H\({}^{+}\) with metal ions for adsorption sites on the PPG-nZVI beads. The increase of pH results in an increase the amount of hydroxyl groups. The number of negatively charged sites on PPG-nZVI beads was improved, leading to the enhanced attraction force between metallic cations and the beads. As a result, the removal efficiency of Pb\({}^{2+}\), Cu\({}^{2+}\) and Zn\({}^{2+}\) by beads increased. On the other hand, metals are immobilized via precipitation by hydrolyzing as metal hydroxides at high pH [4]. Figure 4: Effect of pH (**a**) and ionic strength (**b**) on removal of Pb\({}^{2+}\), Cu\({}^{2+}\), or Zn\({}^{2+}\) by PPG-nZVI beads. From Figure 4b, it is evident that the removal efficiency of Pb\({}^{2+}\), Cu\({}^{2+}\), or Zn\({}^{2+}\) by PPG-nZVI beads was not obviously affected by the presence of ionic strength from 0-0.1 M in NaNO\({}_{3}\) background solution. The result is in accordance with the previous study which indicated that the adsorption of Pb\({}^{2+}\), Cu\({}^{2+}\), or Zn\({}^{2+}\) onto PPG-nZVI beads may be governed by inner-sphere complex adsorption mechanism. In detail, as the concentration of NaNO\({}_{3}\) raised from 0 to 0.003 M, the adsorption capacity of Pb\({}^{2+}\) or Cu\({}^{2+}\) was not affected. However, for Zn\({}^{2+}\), sorption capacity decreased from 2.44 to 2.29 mg\(\cdot\)g\({}^{-1}\).