# Shell Waste Management and Utilization: Mitigating Organic Pollution and Enhancing Sustainability
## 3 Environmental Applications
### Water Purification/Bio-Filters  
It is known that waste shells can be successfully used in wastewater (WW) treatment, particularly in phosphate removal from WW [33]. Phosphorus, along with nitrogen and other nutrients present in WW, leads to eutrophication resulting in algal blooms and oxygen depletion, fatal for aquatic organisms [34]. Adsorption is the simplest and most cost-effective method for the removal of phosphorus, depending on the source of the adsorbent. Waste mollusc shells can act as adsorbents for both industrial and agriculture WW treatment. However, raw shells are less effective at phosphate removal than calcined shells, as they have a lack of stability in acidic environment where they decompose andform sludge, which only adds to the problem of WW [8]. The calcination or pyrolization of shells leads to lime (CaO) formation, which is a common adsorbent, typically obtained from limestone [35]. Limestone needs to be heated to 600 \({}^{\circ}\)C to yield lime, and lime is produced by calcination of shells just as from quarried limestone [36]. Lime dissolves in water to produce calcium hydroxide, which dissociates into calcium and hydroxide ions. These ions react withotrophophates in water and yield calcium phosphate precipitates, one of them being a liming agent hydroxyapatite, the insoluble form of calcium phosphate [8]. Phosphate removal from recirculating marine aquaria is often neglected. However, in seawater, (oyster) shell waste is also a very efficient bioadsorbent for phosphate removal. The main mechanism for raw shells is adsorption, and for calcined shells there is also a co-mechanism of precipitation [37].  
Nitrate removal from WW of fish farms can be attained with woodchip bioreactors enriched with sulfur granules and crushed seashells [38]. Shells contribute by the alkalizing effect on the low-alkaline aquaculture effluents, and compensate for the alkalinity loss from autotrophic denitrification process [39]. Additionally, the presence of crushed oyster shells was shown to accelerate the process of biological nitrification and ammonia removal in synthetic WW, by pH buffering [27].  
Soft calcite, prepared from waste blue mussel shells, can act as an inorganic sponge, which adsorbs dyes from aqueous solution and absorbs crude oil from seawater with good recyclability. Soft calcite has an impressive absorption capacity of 977% \(\pm\) 84% of crude oil and can be used as a high-value material for oil remediation [40].  
The crustacean shell waste proved extremely successful in a number of WW applications. The demineralization and deproteination of crustacean shells yield water-insoluble chitin, with final characteristics of both cellulosic fibers and of proteins, which when deacetylated yield water-soluble chitosan [41]. Both chitin and chitosan are excellent flocculants for pottable water and industrial effluents. They have been applied to remove a variety of water pollutants, such as proteins, lipids, greases, pesticides, PCBs, PAHs, textile dye residues and heavy metals [42].  
The formation of insoluble metal hydroxides by metal adsorption on the surface of calcined shells contributes well to heavy metal removal from water, WW and landfill leachate [43; 44; 45]. It was established that the higher the temperature of calcination, the better the efficiency of calcined shells in heavy metal filtering, related to the increase in the specific surface area upon calcination [9]. The initial water pH, dosage of sorbent, and grain size may affect the heavy metal removal efficiency of shells. The shell powder in aragonite and calcite phase (razor clam and oyster shell, respectively) may remove lead (Pb\({}^{2+}\)), cadmium (Cd\({}^{2+}\)) and zinc (Zn\({}^{2+}\)) from contaminated water [46]. However, the oyster calcite has a higher biosorption capacity to Pb\({}^{2+}\), while the razor clam aragonite has a higher capacity for Cd\({}^{2+}\)[46].