## 1 Introduction  
Over the past decade, industrial and technological developments have transformed the anthropsphere, enabling a breakthrough in productivity. However, these activities may pollute water bodies with various oxyanion contaminants, such as phosphate, arsenate, etc. Arsenate is a well-known carcinogen with a wide range of biological hazards [1]. The ubiquitous occurrence of arsenate is revealed by its contamination in over 70 countries, including Canada, which highlights the global importance of the removal of this waterborne contaminant. Among various water remediation methods such as ion exchange, reverse osmosis, and coagulation-flocculation, adsorption offers flexibility, low cost, low sludge volume, ease of operation, and design advantages [2; 3]. Due to these advantages, phosphate and arsenate adsorption processes that employ environmentally friendly materials are attracting significant interest in the field. In the environment, two main forms of arsenic are present, in the form of arsenite (As(III)) and arsenate (As(V)) oxyanions. Herein, the focus is on the more prevalent arsenate species, As(V). Typical arsenate concentrations are below 0.01 mg/L, main tailings and other high arsenate sources may reach up to 5 mg/L (arsenic) [4; 5]. However, selective removal of arsenic may present challenges in the presence of co-contaminants such as phosphate, which may occur at elevated concentrations (up to 5 mg/L) in aquatic environments [6; 7; 8]. Orthophosphate (P\({}_{\text{i}}\)), in particular, represents challenges concerning arsenate selectivity based on similar pKa values (hence, speciation in water), geometry (tetrahedral), and similar Lewis base hardness [9]. Differentiating traits were identified as size and hydration energy, where arsenate is considerably less hydrated than orthophosphate. The competitive effects of phosphate were summarized by Pincus et al., where metal-based adsorbents (Fe, Al, Ti, etc.) for arsenate observed a 10-90% reduction in As\({}_{\text{i}}\) adsorption capacity in the presence of orthophosphate [9]. For sustainable water remediation, using biopolymer materials becomes ever more important, where two key examples of versatile biopolymers include chitosan (Chi) and alginate (NaAlg). Chitosan is produced by the deacetylation of chitin, a naturally abundant biopolymer found in fungi, shrimp shells, crab shells, and other crustaceans, where its molecular structure is shown in Scheme 1[10]. By comparison, alginate is a natural polymer found in brown seaweed and bacteria that contain \(\alpha\)-L-galuronic and \(\beta\)-D-mannuronic acid monomer units, where its molecular structure is shown in Scheme 2[11; 12]. The abundant functional groups of chitosan (-OH and -NH\({}_{2}\)), and alginate (-OH and -COOH) represent potential complexation sites for metal centers due to their Lewis base character [13]. The relative abundance of chitosan and alginate and their ability to form biocomposites can offer a unique strategy to generate environmentally friendly and low-cost adsorbents with tunable selectivity and adsorption capacity for oxyanions such as phosphate and arsenate. Kumar et al. demonstrated the utility of chitosan and alginate to form polyelectrolyte complexes that contain Al(III) cations, where alginate binds favourably to the Al(III) cations [14]. The resulting material utilizes the biopolymers as scaffolds for the active metal centers, which function as active adsorption sites. Therein, the composites were able to adsorb a variety of contaminants from organic dyes to chromate oxyanions. This research was extended to the design of sulphate adsorbents by Steiger et al. and further extended to other oxyanions such as arsenate and orthophosphate systems [15; 16; 17]. Arsenate adsorption was investigated by employing various aluminum-based ternary metal composites (TMCs), where P\({}_{\text{i}}\) adsorption was probed with TMCs (and binary metal-composites (BMCs) that contain a metal species and biopolymer building blocks). Various metal species may include Fe, Cu, or Al.