Effect of copper (ii) biosorption over light metal cation desorption in the surface of macrocystis pyrifera biomass

Abstract

We study the effect of Cu(II) biosorption on the displacement of intrinsic light metal cations of the surface of the biomass of the brown macroalgae Macrocystis pyrifera. Batch methods and non-linearized kinetic models show chemisorption with a fast reach to equilibrium, represented by a pseudo-second order kinetic model. The non-linear Langmuir isotherm model fits right to the data with a value of maximum capacity of 1.251 mmol g⁻¹ at pH 5. The light metal cations desorption induced by Cu(II) biosorption, show an increase in the desorption of alkali-terrous cations when more Cu(II) is available, especially the desorption of Ca cations. The infrared analysis shows that the functional groups amine, carboxylate, hydroxyl and sulphonate are the main moieties in this biomass. We observed changes in surface texture due to the desorption of light metal cations, and a rearrangement by an apparent increase in fibrillar crosslinking due to the Cu(II) biosorption. The results of the present work show that the cation exchange with Ca and Mg is the primary mechanism in the overall Cu(II) biosorption on M. pyrifera biomass, representing a 56 % of the total of Cu(II) bound.

Introduction

Biosorption is a physicochemical process, where biological material or derivate parts from this material (i.e. biopolymers) are capable of binding cations and anions, due to different physicochemical interactions with ions or molecules to their surface. The study of biosorption phenomenon started in the early 1970s, when radioactive elements (also heavy metals) present in wastewaters, coming from a nuclear-powered station, were found to be concentrated by several algae specimens [1]. Since then, several researchers have demonstrated that biosorption with macroalgae is a promising and cost-effectiveness technology for the removal of heavy metals from aqueous solutions [[2], [3], [4], [5], [6], [7], [8], [9], [10]]. By the other side, biosorption seems to be a structural and complementary response of macroalgae to abiotic stress, as seen in Oedogonium sp, where the increase of ocean acidification by CO₂ affects the bioremediation capabilities of sporophytes [11] positively, so the biosorption turns in an important parameter as indicator of an extracellular matrix reconfiguration [12].

Compared with traditional methods to treat wastewaters like acid mine drainage, as chemical reduction, ion exchange, precipitation, and membrane separation; technologies based on biosorption of brown macroalgae have several advantages: low operative costs, a comparative efficiency compared with traditional methods to detoxify heavy metals in low to medium concentrations, no nutrient requirements by the use of dead biomass, because it seems more effective than use living biomass [7], and a better circular economy [13].

The cell wall of brown seaweeds, that contains a large extracellular matrix (ECM), generally contains three major components: cellulose (structural support) and phycocolloids as alginic acids complexed with light metals such as sodium, potassium, magnesium and calcium, by their constructing blocks (mannuronic and guluronic acids), and fucoidans (sulphated polyfuranoses) [1,14]. These phycocolloids have been reported as the predominant molecular components with active groups who interact, by different physicochemical mechanisms, with light metals and heavy metals [15]. The seaweed biomass possesses several chemical groups that can attract and sequester metals: acetamide, amino, amide, sulfhydryl, sulfonate, and carboxyl [1,16]. This chemical diversity originates a combination of mechanisms to capture the metals, including electrostatic attraction, complexation, ion exchange, covalent bonding, Van der Waals attraction, adsorption and microprecipitation [17]. These properties allows the brown macroalgae to naturally accumulate light metals from their natural niche in their extracellular matrix, due to the affinity of these light metals with many of their structural macromolecules [18]. Considering the importance of light metals in the natural binding capacities of brown macroalgae, the effect of heavy metals, especially which ones have an anthropogenic origin, over light metal mobility is poorly studied in these phycological organisms. Even though the ion exchange is considered the primary mechanism in brown macroalgae biosorption, the exchange of alkali and alkaline-terrous light metals in the biosorption system is poorly characterized. Since these two groups could be non-covalently bound to the same binding sites, so cation exchange data and better mathematical models to interpret these data are necessary [6]. By the technological side, the scarce of information about the exchange of these two light cation metals, by heavy metal biosorption, reduce the chances to develop new products or materials from raw or modified marine biomass, as a competitive technology when are compared to the traditional well-known metal-removal processes [19,20].

Macrocystis pyrifera is part of the group of brown macroalgae from the order of Laminariales (Kelp), with a high presence in the South Pacific Chilean subtidal zones, being one of the primary fisheries resources exported as raw biomass [21]. This kelp has high importance in the formation of benthonic communities and represents a potential biomaterial for biosorption processes, by the high number of sporophytes in the adult stage, with a high availability of biomass that allows the obtainment of derived products, including biosorbent materials [22]. The present study was focused on the effect of Cu(II) biosorption over light metal cation biosorption naturally present on brown M. pyrifera biomass surface, in order to understand the dynamics of alkaline and alkaline-terreous light metals desorption, to develop biosorbents in a more rational way, using Cu(II) as a model of desorption dynamics. Hence, the novelty of this study is the use of M. pyrifera dried biomass as a biosorbent to remove Cu(II) ions in aqueous solution, with the simultaneous desorption of light metal cations. Data from this single metal biosorption will be useful to further study the biosorption process in multi-metallic systems, as usually can be found in real wastewaters, with the considerations of the light metal cation desorption. Biosorption kinetics and isotherms by non-linear equation models were studied, as the pH influence over Cu(II) biosorption and on the desorption was studied by batch methods. The critical functional groups present in this biomass was also studied. Finally, the morphological changes induced by biosorption over the biomass surface was observed by microscopical methods.