## 1 Introduction  
Water pollution is a phenomenon that has a negative impact on both human health and ecosystems [1]. The most polluting dyes allowed are mainly used in the leather, plastic, mineral oil, and cosmetic wax industries [2], as well as in other industries such as food, paper, textiles, and pharmaceuticals, with an annual output of \(7*10^{5}\) tons [3]. Thesedyes are harmful, carcinogenic, mutagenic, toxic, and harmful to human health, and they undoubtedly harm the environment [4].  
In previous years, there have been several works on the removal of dyes by biological and physicochemical treatments, including coagulation, flocculation, filtration, advanced oxidation, ion exchange, membrane processes, aerobic and anaerobic degradation [5], membrane distillation [6], electrochemical methods [7], irradiation [8], Fenton reaction [9], reverse osmosis, precipitation, ozonation [1], and adsorption [10]. These treatments are effective yet highly costly, limited in efficiency, and deficient in process selectivity [11]. In addition, other hazardous byproducts are formed and require further treatment, with consequent energy consumption [12].  
Compared to other water treatment processes, adsorption is one of the simplest, safest, and most versatile techniques to retain the pollutants [13]. It is the most efficient technique, with minimal use of chemicals, and it is easier to implement in wastewater treatment plants [14].  
Bio-adsorption is an approach that is generally applied to eliminate heavy metals [15] and different types of organic pollutants [16]. The use of various alternative adsorbents can be both effective and less expensive in removing organic pollutants, such as nutrients and dyes, according to research [17]. For example: rice hus [18], orange and banana peels [19], bark, pineapple stem waste, water hyacinth pulp powder, tuberous pulp, sugarcane pulp, coconut pulp [20], tea waste, com cob, wheat bran, peanut skin, bamboo activated carbon, modified corncobs, modified cassava waste, lemon peel, durian peel [21], pomelo peel [22], potato peel [23], chitosan, zeolite, and activated carbon [24].  
Cockleshells have become a major source of protein in Southeast Asia and a source of calcium carbonate (CaCO\({}_{3}\)) in Malaysia. The strong demand for marron-based seafood has generated a considerable quantity of shellfish. A number of studies have been conducted to utilize waste cockleshells as valuable materials. For example, the mineral content of CaCO\({}_{3}\) in shell waste can be converted into lime to be used as a heterogeneous catalyst in biodiesel production, biolubricant production, and also as an adsorbent in wastewater treatment processes [25].  
Cockleshells may be considered to be an inexpensive source of CaCO\({}_{3}\). They are composed of CaCO\({}_{3}\), mainly as a mineral calcite or aragonite. CaCO\({}_{3}\) is gaining a lot of attention for its role as a heavy metal adsorbent. CaCO\({}_{3}\) is a must for absorbents because it can be regenerated with low-cost HCl to desorb heavy ions and maintain high adsorption capacities after multiple cycles. Using cockleshells as an adsorbent could represent an economic and environmental way forward, as the process is highly efficient and uses low-cost materials for the exploitation of aquaculture waste products to achieve a zero-waste production system [26].  
Cockleshells, which are natural marine waste, were used as bio-adsorbents in this research.  
The cockleshells were evaluated for removing Direct Red 81 from aqueous solutions.  
The effects of time, initial concentration, mass of bio-adsorbent, temperature, and solution pH were studied to find the optima of these effects on the adsorption of DR-81. Kinetics, thermodynamics, and optimization by Box-Behnken design were studied.