The older and principal market for shells waste lies in the agricultural sector involving the neutralization of acidic soils. Generally referred to as liming, this practice involves treating soil or water with lime (or a similar substance) in order to reduce acidity and improve fertility and oxygen levels. In acidic soils, shell waste seems to be a practical and interesting approach to suppress the needs in terms of calcium and provide good conditions for pH adjustments (i.e., liming), while promoting the goal of developing a circular economy and environmental sustainability ^[1][2][29,30]. Historically, shells from mussels (Mytilus galloprovincialis) were used after thermal treatment as a liming agent or as mulches for soil amendments in farming in Galicia ^[1][29]. Calcium carbonate (CaCO₃) is reported to neutralize acidic soil and increase soil organic matter, available P, and exchangeable cations concentrations, while improving its fertility and increasing oxygen levels ^[3][26].

Alvarez at al. ^[4][31] compared the effects of different types of mussel shell waste and commercial lime on soil and plant production. They found that the efficiency of treated mussel shell waste was comparable to commercial lime, with regard to the increase in pH, soil characteristics and improved pasture quality, increasing the Ca concentration in the plant tissues and favoring Ca and K absorption by plants. Lee et al. ^[5][32] carried out an environmental impact analysis based on a life cycle assessment of using oyster shell waste as a liming agent compared with the use of eggshell, demonstrating the major impact derived from electricity consumption on the calcination, milling and the drying processes.

Bivalves’ shell waste has also been used as a liming agent, combined with other substances to improve soil fertility. For example, oyster shells were treated with coffee waste effluents and used as a soil fertilizer ^[6][33]. The high alkalinity, which contributes to a reduction in soil acidity, with the presence of coffee pulp, which provides soil fertility richness by means of high levels of organic matter, nitrogen, and potassium, makes it a valuable organic fertilizer. Furthermore, Kwon et al. ^[7][34] reported that blending 10–30% of powdered oyster shell blended with sewage sludge has positive effects on soil stabilization and fertilization. Paz-Ferreiro et al. ^[8][35] demonstrated an enhancement in soil chemical and biological parameters using mussel shells alone or in combination with cow slurry.

Moreover, crushed shells represent an important calcium supplementation (CaCO₃) when introduced into livestock feeding. Calcium supplementation is used to improve the health of livestock, particularly bone health, but also in laying birds as a supplement to improve the quality and strength of eggshells ^[9][36]. The replacement of the calcium present in limestone with that from oyster shells was proven to enhance bone development, egg production, and the strength, weight and thickness of eggs in hens ^{[10][11][12][13]}[37,38,39,40] and ducks ^[14][41]. Another recent study found that oyster shells alone performed better than snail shells; wood ash or limestone, as a calcium supplement in terms of growth response; and weight gain and feed intake in laying hens ^[15][42].

According to the EU Regulation (EC) No 1069/2009, shell waste can be used as a feed supplement only if it is free from flesh, in order to be exempt from animal by-product classification ^[16][18]. The attribution of free-from-flesh standards is regulated by the respective authority of each member state. Eventually, for this type of valorization, the distance between shell production and farms must be taken into account from both an environmental and economic sustainability perspective ^[17][43].

The use of shell waste as amendments to remediate heavy metals in contaminated soil has attracted growing research interest and been widely applied ^[18][19][44,45]. Calcium carbonate increases soil pH, leading to the formation of calcium silicate hydrate and calcium aluminate hydrate, which create a relatively impermeable soil layer and decrease the mobility of heavy metals ^[20][46]. In addition, shells are promptly available and cost-effective materials, making them a desirable solution when vast amounts of contaminated soils must be remediated, including those in mining areas ^[21][47].

Oyster shell waste was applied for the treatment of cadmium- (Cd²⁺) and lead (Pb²⁺)-contaminated soils near a closed mine. Moreover, an improvement of soil nutrients by applying oyster shell powder containing Ca²⁺, Na⁺, Mg²⁺ and K⁺ was reported ^[22][48].

Zhong et al. ^[23][49] reported the potential use of waste oyster shells as adsorbent and amendment agents for effective metal immobilization in both aquatic and sediment systems. Chen et al. ^[24][50] reported a significant reduction in arsenic (As³⁺) leachability in highly contaminated soils when 2% oyster shell waste was applied in combination with 2% biochar, preventing shallow aquifer pollution, especially in sites with frequent precipitation. The use of 2% calcinated oyster shells was able to reduce the exchangeable Cd²⁺ in soil by 55%, and by 98% and 3% Cd²⁺ and As³⁺ content in the edible part of vegetable crops, making them safe for consumption ^[25][51]. A mixture of calcined oyster shells and waste cow bones was employed to remediate Pb²⁺ and copper (Cu²⁺) in army firing range soils ^[26][52], reducing the availability of Cd²⁺, Pb²⁺, Cu²⁺ and zinc (Zn²⁺) metal-contaminated soil ^[27][53].

In some cases, crushed and calcined mussel shells were used for soil remediation because they were rich in carbonates, oxides and different functional groups, with higher particle sizes and surface areas that could readily immobilize heavy metals ^[28][54]. Additionally, mussel shells could also increase soil pH and reduce the solubility of metals. Mussel shells drastically decreased Cu²⁺, Cd²⁺, Zn²⁺ and nickel (Ni²⁺) mobility and the availability of plant uptake, reducing the overall risks of soil and water pollution ^[29][30][31][55,56,57].

The bioavailability of Pb²⁺ in military shooting range soils strongly decreased by 92.5% and 48.5% in mussel shells compared to the unamended soil, while decreasing the risk of ecotoxicity and supplying the soil with phosphorous ^[32][58].

Adsorption has been reported as an attainable treatment process to eliminate traces of heavy metals from water, wastewater and landfill leachate ^[33][34][59,60]. Other conventional methods are currently widely employed, such as precipitation or ion exchange, but they are not always practical due to their high cost of operation and maintenance requirements, especially in developing countries ^[35][61]. Mollusk shells have recently been proposed as possible alternative biomaterials for heavy metal absorption ^[36][62]. They have been proposed for the treatment of electroplating wastewater from copper-mining industries in Guangxi of China ^[37][63]. Raw and acid-pretreated bivalves’ shells demonstrated the efficient removal of Cu²⁺ through metal absorption based on the ion exchange between Cu²⁺ in the treated solution and Ca²⁺ from the shells. Nano-scale mollusk shell powders in aragonite were shown to effectively function as adsorbents in the treatment of Pb²⁺-, Zn²⁺- and Cd²⁺-rich wastewater derived from plastics manufacturing, batteries, electroplating, metalworking, and mining industries ^[38][64]. Oyster shells have been used for removing Cd²⁺, chromium (Cr^3+/6+) and cobalt (Co²⁺) from contaminated waters ^[39][40][41][65,66,67] and As³⁺ from tube well water ^[42][68] and mine tailings ^[43][69]. Unprocessed whole oyster shells have also been investigated for removing Cu²⁺, Zn²⁺, Cr⁶⁺ and Cd²⁺ in stormwater management infrastructure, with a good efficiency against Cu ions (80–95%), Cd ions (50–90%), Zn ions (30–80%), and hexavalent Cr (20–60%) ^[44][70]. Calcinated and raw mussel shells showed a high capacity of mercury (Hg²⁺) retention ^[45][71] and were recently studied for phosphate removal from water, yielding very promising results: above 90% ^[46][47][48][72,73,74].

Lately, mollusk shells have also been used in combination with waste glass as absorbents for removing the direct blue 15 azo dye ^[49][50][75,76], methylene blue ^[51][77], and red dye ^[52][78] from industrial wastewater. Moreover, they have been proposed as absorbents for the removal of emerging contaminants, such as hormones, personal care products, pharmaceutical products and pesticides, from domestic and industrial wastewaters, obtaining efficiency values higher than 30% ^[53][79]. There were some studies from  recent publications where shells waste were recommended for their removal of rifampicin antibiotic ^[54][80] and biocides residues from waters, such as triarylmethane ^[55][81]. Powdered mussel shells were used for the removal of methyl blue and methyl red, together with Cr⁶⁺, Cd²⁺ and Cu²⁺ from wastewater, demonstrating a capacity of almost 100% removal, as well as improving the settling properties of the activated sludge flocs ^[56][82].

Concrete—a mixture of gravel, sand, cement, water, and mortar formed by cement and sand—is among the most widely used building material. The concrete and mortar industry has harmful effects on the environment, especially in terms of greenhouse gas emissions. The use of mollusk shell waste as a substitute for concrete and mortar ingredients is attracting research interest as a viable alternative that may reduce our dependency on conventional components ^[57][58][83,84]. In order to evaluate the possible application of bivalves’ shells in the production of calcitic lime, Ferraz et al. ^[59][85] characterized shells of eight different species of bivalves for their mineralogical, chemical and thermal properties, compared with commercial limestone as reference. It was noted that scallop and oyster shells are composed mainly by calcite, similar to the reference limestone, whereas mussel shells mainly contain calcite. Additionally, aragonite, edible cockle, wedge, razor clam, dog cockle and clam shells contained aragonite as principal component, with minor or negligible amounts of calcite.

Researchers generally suggested that a few important steps of treatment should be considered prior to reusing seashell waste: washing, calcining, and crushing to the desired size. As seashells are primarily obtained as waste, the proper handling and treatment of the waste must be carried out before they are incorporated in concrete, to ensure the removal of impurities. Additionally, heating at 500–650 °C is required to remove the organic matter attached to the seashell, and crushing is needed to ensure better bonding between the seashell aggregate and the cement. Several studies already investigated the opportunity of integrating bivalve shells in ordinary ^{[60][61][62][63]}[86,87,88,89] and pervious ^[64][65][66][90,91,92] concrete, mortar ^{[67][68][69][70]}[93,94,95,96], air lime mortar ^[71][72][97,98], artificial stone ^[73][74][99,100], and bricks ^[75][76][77][101,102,103], in a broad range from 5 to 90% ^{[78][79][80][81]}[104,105,106,107]. However, generally speaking, it was reported that the inclusion of bivalve waste as an aggregate in mortar or concrete reduced its strength and workability properties by more than 20% ^[82][108]. Due to these difficulties and the strict safety regulations in building materials, the wide-scale application of mollusk shells in construction is not yet an established market ^[16][18]. Alternatively, there are many historical examples of shells in residential houses or walls in different coastal areas ^[83][109].

Being rich in calcium carbonate, when calcined at a proper temperature, shells are converted into metal oxide CaO, which can be used as a heterogeneous catalyst in the transesterification process of biodiesel production ^[84][110]. Chemically, biodiesel contains alkyl esters of long-chain fatty acids derived from renewable feedstocks, such as vegetable oils and animal fats ^[85][111]. Transesterification with methanol is one of the most utilized routes for first-generation biodiesel production, consisting of a chemical reaction of vegetable oil/animal fat with alcohol in the presence of a base catalyst to form glycerol and esters ^[86][112]. Even though homogeneously catalyzed biodiesel production processes are relatively faster and show higher conversions, heterogeneous catalysis using CaO is becoming more popular because it overcomes some disadvantages of the traditional method, namely side-reactions leading to soap production, and the difficulty of separating the catalyst from products at the end of the reaction, thus leading to high costs ^[87][113]. Hu et al. ^[88][114] showed how calcinated and rehydrated mussel shells could be used as catalysts for palm oil transesterification in five tests of reuse with a yield of 95% in the first test and 59.1% in the last test. The negative effects of the reusability of shells on biodiesel yield were also confirmed by Rezaei et al. ^[89][115]. Mussel shells were used as catalysts in transesterification, achieving the following yields for a number of substances: methanol of palm oil, nearly 95% yield ^[90][91][116,117]; castor oil, 91.17% yield ^[92][118]; sunflower oil, 90–97% yield ^[93][119]; jatropha oil, 97.54% yield ^[94][120]; and peanuts and rapeseed oil, 94 and 96% yield, respectively ^[95][121].

Biodiesel from camelina oil was obtained using mussel, clam and oyster shells with 95, 93 and 91% yields, respectively ^[96][122].

Clam shells were used in both first-generation biodiesel from rapeseed ^[97][98][123,124], jatropha ^[99][125] or cottonseed ^[100][126] oils and second-generation biodiesel production in the transesterification of waste frying oil, with a conversion of 94.25% compared with 67.57% obtained using commercial CaO ^[101][127].

KI-impregnated and calcinated oyster shells were used as solid catalysts for the transesterification of soybean oil with an 86% yield ^[102][128]. Combusted oyster shells were exploited in soybean oil transesterification under the optimum reaction conditions and reached a 73.8% yield with a high biodiesel purity (98.4 wt.%) ^[103][129]. Crude bio-oil was extracted from the seeds of capers and transesterified into biodiesel using CaO from oyster shells ^[104][130]. Oyster shells were also employed for biodiesel synthesis from waste cooking oils, achieving an 87.3% yield ^[105][131].