# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Types of Adsorbent
#### 6.6.4 Seaweed and Alginate  
Seaweed contains a large amount of metal-sorbing biomass. Some seaweed having polysaccharide content was recognised for the ion exchange properties. These properties were especially noticeable in brown algae and the highest sorption amount was found to be about 67 mg of cadmium per gram of seaweed [109]. Furthermore, _Ulva lactuca_ green algae were less effective in comparison to brown seaweeds for adsorption of cadmium and mercury removal. Even though seaweed has a high sorption capacity, they have a property to break down materials as well, and therefore they were modified through crosslinking with glutaraldehyde, formaldehyde, and polyethyleneimine [110]. In algae, crosslinking can enhance the firmness and mechanical properties of the algae. _A. nodosum_ was found to adsorb 215 mg cadmium per gram of raw seaweed and 149 mg cadmium per gram of formaldehyde-crosslinked seaweed. A. nodosum was more viable as compared to other seaweeds tested. [PERSON] et al., prepared different sizes of two marine algae, _Sargassum fluitans_ and _Ascophyllum nodosum_, for the extraction of toxic metal ions like cadmium, copper, lead, nickel, and zinc [139]. Alginate, a natural biopolymer, is also utilised as an adsorbent to remove metals including Pb, Ni, Hg, and Cu, as well as organic contaminants, from wastewater. Alginate is a structural component of marine brown algae such as _Macrocystis pyrifera_, and it is abundant in nature. Because of its electrostatic interactions, this polysaccharide has several anionic or cationic groups in its structure, giving it distinctive physical features. Alginate is made up of 1,4-linked D-manuronic and L-galuronic acids. Alginate is an anionic polymer found in the outer cell walls of brown algae such as kelp. Alginate's major component is alginic acid, whereas sodium alginate (SA) is the sodium salt of alginic acid, having numerous free hydroxyl and carboxyl groups scattered along the polymer's backbone chain. When Ca\({}^{2+}\) is added to the SA solution, it displaces some of the H\({}^{+}\) and Na\({}^{+}\), resulting in the formation of calcium alginate (CA) gel. The capacity of sodium alginate to bind to multivalent cations follows the following sequence: Pb\({}^{2+}\)\(>\) Cu\({}^{2+}\)\(>\) Cd\({}^{2+}\)\(>\) Ba\({}^{2+}\)\(>\) Sr\({}^{2+}\)\(<\) Ca\({}^{2+}\)\(>\) Co\({}^{2+}\)\(>\) Ni\({}^{2+}\)\(>\) Zn\({}^{2+}\)\(>\) Mn\({}^{2+}\)[140].  
[PERSON] et al. investigated As removal using three different adsorbent materials: KOH-activated carbon-based apricot stone (C), calcium alginate beads (G), and calcium alginate/activated carbon composite beads (C) (GC). GC showed the maximum As (V)  
Figure 6: Adsorption mechanism diagram and contributions of adsorption Pb\({}^{2+}\) and Cu\({}^{2+}\) onto XCMCP [138].  
adsorption (66.7 mg/g at 30 \({}^{\circ}\)C), according to the data [141]. To absorb Cr (VI) and As (V) from wastewater, [PERSON] et al. created magnetite GO encapsulated in calcium alginate beads (mGO/beads) [142]. [PERSON] et al. entrapped silica nanopowders within calcium alginate and found that a pH of 5.0 was best for Pb (II) adsorption, with a maximum adsorption capacity of 83.33 mg/g [143]. For example, [PERSON] et al. found that Ca-alginate beads with numerous oxygen-enriched functional groups (-COOH/-OH) had a reasonably high capture capacity for Pb\({}^{2+}\) (360.11 mg/g) [144].