Application of ecofriendly cation exchangers (Gracilaria caudata and Gracilaria cervicornis) for metal ions separation and recovery from a synthetic petrochemical wastewater: Batch and fixed bed studies

Red macro-algae Gracilaria caudata and Gracilaria cervicornis were used as cation exchangers for transition metals (copper, nickel and zinc) from a synthetic petrochemical wastewater. Scarce information is available in the literature about the use of red algae as natural cation exchanger and the effect of multicomponent aqueous matrix on the treatment performance. In the present work, the transition metals separation and recovery were evaluated considering the matrix effect. The ion exchange capacities for both red algae were near 1.2 mEq/g, which corresponds to the total amount of light metals present on the surface of raw algae. A heterogeneous model, considering 3 inflection points, was developed to enable the identification and quantification of the functional groups present on the biomass surface and the results obtained are in agreement with the total amount of light metals obtained by biomass digestion. Equilibrium and kinetic experiments were conducted using both raw algae and calcium loaded G. cervicornis. Cation exchange equilibrium was described by a Langmuir multicomponent model. The equilibrium affinity constants for the functional groups increases in the following order: nickel < zinc ≪ copper. G. cervicornis showed a higher selectivity for the transition metals present in the synthetic petrochemical wastewater and it was used in the following tests. Calcium loaded G. cervicornis showed a higher affinity to copper than for zinc and nickel. Biomass regeneration was evaluated using several eluents (ultrapure water and 0.1 M solutions of hydrochloric acid, nitric acid, sulfuric acid, sodium chloride, calcium chloride and sodium hydroxide) and regenerant solutions (0.5 M solutions of calcium chloride and sodium chloride). Cation exchange breakthrough curves obtained from a column packed with raw G. cervicornis, led to a useful capacity of 0.25 and 0.24 mEq/g for the first and second operation cycles. A mass transfer model, considering a linear driving force model for intraparticle diffusion, was able to predict well the ion exchange process for all species studied at batch and packed bed systems. A treatment strategy using a column system in series was proposed to increase the treated water volume using G. cervicornis as a natural cation exchange resin, being possible the recovery of transition metals from separate solutions.