# Adsorption of Ni2+ and Cd2+ from Water by Calcium Alginate/Spent Coffee Grounds Composite Beads
## 1 Introduction
Finally, the research in low-cost adsorbents can lead to creation of novel low-costs adsorbents with the addition of innovative materials such as titanium dioxide (TiO2) to the basic structure of the low-cost adsorbents with the capacity to remove different kinds of pollutants in a single step (heavy metals and surfactants) [11]. The use of waste material as adsorbent of pollutants represents a double advantage over the use of other adsorbents because beside the removal of the pollutant, a recycling of waste material is being carried out. The coffee industry is responsible for the production of more than 2 billion tons of by-products such as spent coffee grounds (SCGs) [12]. The coffee spent grounds are a pollutant by themselves and most of them are wasted in landfills. Spent coffee grounds are discarded into landfills and they create methane, which is a greenhouse gas [13]. The chemical composition of SCGs is based on hemicellulose, cellulose, fat, proteins, polyphenols, minerals, and products from Maillard reaction during the roasting process, such as melanoidins [13]. Major components of the SCGs, namely polysaccharides cellulose and hemicellulose, and the polymeric compound lignin can be used to remove heavy metals from water because they have functional groups as \(-\)OH, \(-\)COOH and phenolic groups which are able to adsorb heavy metals from aqueous solutions. SCGs have been used for the removal of heavy metals from water [14]. Alginic acid or alginate is an acidic polysaccharide from the cell walls of brown algae. Usually it is used as calcium alginate (CA) that is insoluble and created with the addition of an aqueous solution of sodium alginate to an aqueous solution of calcium chloride. During the reaction, Na\({}^{+}\) ions are replaced with Ca\({}^{2+}\) ions to create the insoluble CA. CA beads have been used for the adsorption of heavy metals, due to their good affinity for the bivalent metal ions [15]. Alginic acid is composed of \(\beta\)-D manuronic acid(M) and \(\alpha\)-L-glucuronic acid (G), but only the G-blocks of alginate are believed to participate in intermolecular cross-linking with divalent cations (e.g., Ca\({}^{2+}\)) to form hydrogels [16]. SCGs have been encapsulated before in CA in our previous study, showing greater adsorption capacity for Cu\({}^{2+}\) in water and improved kinetics than CA alone at concentrations between 10 to 100 ppm of copper at pH 4 and 30 \({}^{\circ}\)C [17]. In the previous study, the larger adsorption by CA-SCG beads in comparison to the CA beads was seen in the lower range of concentrations (10 to 30 ppm). At these concentrations, the percentages of adsorption were closer to 100% versus the CA alone, approaching the goal of total metal removal from the water [17]. Most of the contamination by heavy metals in water is at levels of trace concentrations. Adsorbents with a capacity of total metal removal at a lower concentration are more significant than adsorbents with larger capacities of adsorption but that cannot remove all the dissolved metal no matter the concentration, taking into consideration that heavy metals can be dangerous even at low concentrations. The analysis of the CA beads prepared in this study focuses the performance of this matrix alone versus the composite beads (CA-SCGs beads) and the SCGs alone. The synthesis of CA beads was kept simple without any further step to improve the adsorption of metals besides the addition of SCGs. CA beads and CA-SCGs beads were prepared under the same conditions. The aim of this research was to study the performance of the CA-SCGs beads in the removal of Ni\({}^{2+}\) or Cd\({}^{2+}\) from water at concentrations from 10 ppm to 100 ppm.