Acronym: WHIERTARN
Wastewater Heavy ion metals decontamination with Ion
Exchange Resins: TARNiţa closed mine pollution case
The resins were deeply investigated in terms of chemical composition (anionic, cationic, amphoteric) and cross-linking degree, by: swelling degree, ion-exchange capacity, volume variation, charge density, size.
Summary of stage 2/2021-Synthesis and characterization of acrylic ion exchangers and batch sorption experiments of heavy metal ions from synthetic waters and from Tarnița wastewater.
The second stage was focused on the synthesis and characterization of some ion exchange resins based on acrylic copolymers (to obtaining a library of ion exchange resins based on acrylic copolymers) for the use in subsequent batch sorption experiments and identification of the suitable ion exchange resins to be used in subsequent column-based experiments. Cross-linked acrylic copolymers, as beads, were obtained following an already published procedure, by aqueous suspension radical polymerization. Briefly, different ratio between divinylbenzene, DVB (as crosslinker) and ethyl acrylate, EA and acrylonitrile, AN (as comonomers) were used, namely: 3% DVB, 20% AN, 77% EA and 8% DVB, 20% AN, 72% EA, obtaining the DEAn copolymers, with n = 3 or 8 (n corresponding to the DVB content). The reaction took place in the presence of toluene as an inert component [D = 0.4, mL diluent/(mL diluent + mL comonomers)]. Ammonium salt of poly(styrene-co-maleic anhydride) was used as stabilizer in the aqueous phase (0.5%) and benzoyl peroxide (1%) was used as polymerization initiator.
Next step was obtained of a library of ion exchange resins as beads with multi-channeled, having weak acid (by basic hydrolysis of acrylic copolymers with sodium hydroxide), basic (by aminolysis with ethylenediamine (EDA) and triethylenetetramine (TETA), reaction with hydrazine hydrate (HA) and with hydroxylamine hydrochloride (HAm). Ion exchangers obtained with EDA and TETA can be further transformed by adding 4-dimethylaminobenzaldehyde) and amphoteric functional groups (carboxymethylation reaction with sodium chloroacetate of weak cationic ion exchangers) with high affinity to heavy metal ions. The functionalization pathway for obtaining the all the ion exchangers is summarized in Scheme 1.
The qualitative evaluation of the chemical modifications has been performed by FTIR-ATR spectroscopy, to get information about the structure of obtained ion exchange resins. FTIR-ATR spectroscopy demonstrated the presence of the functional group in the acrylic copolymers, with the formation of functionalized acrylic ion exchange resins.
The surface morphology of the ion exchange resins based on acrylic copolymer has been evidenced by SEM. As shown in figure, the surfaces modifications after copolymers functionalization depends on the crosslinking degree of the precursors, increasing the DVB content to 8% leading to more compact structures which kept the morphology after functionalization, irrespective of the amine used in aminolysis process (EDA or TETA). The bead structure after the introduction of functional groups strongly influenced the initial structure morphology, at lower crosslinking density (3% DVB).
The obtained ion exchangers were tested as sorbent for heavy metal ions (mixed solution) in batch experiment from synthetic wastewater and wastewater from Tarnița.
To evidence the color changes after and before the heavy metal ions sorption protocol, photographs of the acrylic exchange beads beads are presented below.