Reactivity and Heavy Metal Removal Capacity of Calcium Alginate Beads Loaded with Ca–Al Layered Double Hydroxides

Andres Borgiallo,Ricardo Rojas

doi:10.3390/chemengineering3010022

Abstract

Layered double hydroxides (LDHs) present multiple applications due to their versatility and reactivity. Thus, Ca–Al LDHs with Friedel’s salt structure (HC) have been proposed as heavy metal scavengers due to their buffering capacity at basic pHs. Nevertheless, the control of the reactivity of LDHs such as HC is necessary to optimize their applications. Here, the reactivity of an HC prepared by a coprecipitation method was modified by its inclusion in calcium alginate (CaAlg) beads prepared by ionic gelation. The obtained beads (CaAlg/HC) showed good dispersion of the HC particles in the alginate matrix and were used to test the acid base reactivity and heavy metal uptake capacity compared with pure CaAlg beads and HC powder separately. The pH buffering capacity of CaAlg beads was enriched by the inclusion of HC that, in turn, was modulated in its reactivity. Thus, the HC dissolution times changed from mere seconds for the powder to tens of minutes when enclosed in the beads in a kinetic profile determined by the diffusive step. On the other hand, Cu2+ uptake capacity of CaAlg/HC beads combined the Cu(OH)2 precipitation capacity of HC with the complexation capacity of alginate, reaching good affinity and capacity for the obtained beads. Nevertheless, the precipitation of the hydroxide was produced outside the bead, which would induce the addition of an additional separation step to produce an acceptable Cu2+ elimination.

Highlights

The combination of natural polymers and inorganic materials has drawn considerable attention due to the unique properties of the composite materials obtained, which makes them attractive in applications as packaging films, scaffolds, filters, etc
The above described bead preparation methods allowed the obtaining of pure calcium alginate (CaAlg) and Ca–Al layered double hydroxides (LDHs) containing (CaAlg/hydrocalumite structures (HCs)) beads with regular sizes (Supporting Information, Figure S1): the former ones were more transparent than the latter, which showed a white shade compared to the yellowish-brown CaAlg beads
The CaAlg/HC beads were prepared right after the NaAlg/HC mixture was prepared, as a viscosity increase was observed with time

Summary

Introduction

The combination of natural polymers and inorganic materials has drawn considerable attention due to the unique properties of the composite materials obtained, which makes them attractive in applications as packaging films, scaffolds, filters, etc. Alginic acid is a natural, linear polymer that can be described as homopolymeric blocks formed of D-mannuronate and L-guluronate units. It portrays a large content of carboxylic groups, mainly deprotonated at neutral pH, and the interaction with Ca2+ ions leads to the crosslinking of the alginate chains and the formation of a water-entrapping calcium alginate network. This process is used to form calcium alginate beads by the simple dropping of a sodium alginate solution in a CaCl2 solution [5,6]. The presence of carboxylate groups allows the uptake of heavy metal ions, such as Cu2+, Pb2+, or Cd2+, incorporated by chelation and ion exchange mechanisms [1,7,8]

Methods

Results

Conclusion