Boron removal by a composite sorbent: Polyethylenimine/tannic acid derivative immobilized in alginate hydrogel beads

Abstract

A novel composite material was prepared by the grafting of tannic acid on polyethylenimine (PEI), which allows an efficient sorption of boron (sorption capacity close to 0.89 mmol B g). The encapsulation of this chelating sorbent (finely crushed) facilitates its use (readily solid/liquid separation, use in fixed-bed columns) at the expense of a loss in sorption capacity (proportionally decreased by the introduction of alginate having poor efficiency for boron uptake). Sorption isotherms are modeled using the Langmuir equation, while the kinetic profiles are presented a good fit by pseudo-second order rate equation. In addition, the encapsulating matrix introduces supplementary resistance to intraparticle diffusion, especially when the resin is dried without control: freeze-drying partially limits this effect. The stability (at long-term storage) of the sorbent is improved when the sorbent is stored under nitrogen atmosphere. The presence of an excess of NaCl was investigated. The degradation of the hydrogel (by ion-exchange of Ca(II) with Na(I)) leads to a decrease in the sorption performance of composite material but the action of Ca(II) ions in the solutions re-stabilizes the hydrogel.

Adsorption mechanism of borate with different calcined layered double hydroxides in a molar ratio of 3:1

“…Because the ionic sizes of B 4 O 5 (OH) 4 2are larger than OHand B(OH) 4 -, Zn-LDH (3:1) with larger layer distances are formed after 8 h as shown in the XRD pattern (Fig. 3), and the corresponding diffraction peak intensity increases slowly as the reaction occurs.…”

“…Through the above analysis, the following conjecture can be made. Because of the structure regeneration of Zn-CLDH (3:1), B(OH) 4 and B(OH) 3 enter the interlayer, but a large number of electrostatic adsorption sites gradually increases the size of interlayer ions, resulting in the partial polymerization of boric acid. The borates are gradually adsorbed in a polymer and free form.…”

“…At the beginning of reaction, the major boron species have two forms of B(OH) 4 and B(OH) 3 when the boron concentration is lower than 25 mmol/L [18]. Therefore, the following reaction occurs by electrostatic adsorption [Eq.…”

“…Therefore, at initial pH 7 and when the borate concentration is 2.5 mmol/L, B(OH) 4 -, and B(OH) 3 are adsorbed on the oxide surface, which is the first step to remove borate. Mg-CLDHs dissolves Mg 2+ and Al 3+ and simultaneously produces OHat the beginning of reaction, as shown in Eqs.…”

“…As one of the essential micronutrients, boron is important for plants, animals, and humans [1][2][3]. However, in recent years, boric acid has been widely used as an important raw material in various industries, releasing a large amount of boron compound into the environment [4]. In addition, the removal of borate in seawater for production of drinking water is difficult and expensive because of its excellent water solubility [5,6].…”