Brown macro-algae as natural cation exchangers for the treatment of zinc containing wastewaters generated in the galvanizing process

•Macroalgae as natural cation exchangers for the treatment of galvanic wastewater.•Raw Laminaria hyperborea showed a higher affinity for zinc from the galvanic effluent.•The zinc ions were easily displaced from the exhausted biomass using acid eluents.•The regenerated Ca-loaded alga showed similar performance to raw Ca-alga.•Na-loaded alga showed severe physical deterioration due to alginate solubilization.

In this study, brown macro-algae harvested from the north coast of Portugal, Laminaria hyperborea, Fucus spiralis, Pelvetia caniculata and Ascophyllum nodosum, were used as natural cation exchangers for the treatment of zinc-containing rinse waters generated in the galvanizing process. The zinc wastewater is characterized by a high conductivity (1.5 mS/cm), a low organic content and a zinc concentration of 10 mg/L, being approximately 80% and 20% in the form of Zn2+ and ZnSO4(aq), respectively.Although the ion-exchange capacity of the four macro-algae is similar (2.2–2.4 mEq/g), mainly associated with the presence of weak acidic carboxylic and strong acidic sulfonic groups, L. hyperborea showed a higher selectivity for zinc ions. The uptake capacity for zinc ions is highly affected by the galvanic wastewater matrix, when compared with the performance using a pure zinc solution. Zinc uptake equilibrium, on all the natural cation exchangers, was well described by a linear relationship. Cation exchange kinetic was adequately represented by a mass transfer model, considering a linear driving force model for intraparticle diffusion. Na-loaded alga showed a higher affinity for zinc ions when compared with Ca-loaded alga and raw alga. HCl, HNO3, H2SO4 and CaCl2 eluents were tested for zinc displacement from exhausted algae. HNO3 reached an elution efficiency of 100% in less time than the other eluents tested. After the acid elution, the resin was converted to Na or Ca form (Na- or Ca-H-loaded biomass), using NaCl and CaCl2 solutions. Although the regenerated alga in the form of sodium showed a higher affinity for zinc ions, its physical deterioration after Na loading may affect its application in a full-scale treatment.

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