**1. Introduction**

Industrial development is a significant contributor to a country's financial status, including using dyes to produce an eye-catching product that meets the consumers' demand [1]. Estimated that 10–15% of dyes used in textile processing were lost in the wastewater. Coincidentally, methylene blue (MB), a sulfur-containing heterocyclic aromatic dye, is mostly applied in textiles industries for dyeing cotton, silk, and wool [2, 3]. In addition, other applications in chemistry, biology, and medical science also use this basic cation dye in the treatment of methemoglobinemia and cyanide poisoning [4].

Biological and chemical precipitation is the standard dye removal treatment [5] while several conventional wastewater treatment methods have also been tested for the same purpose. Coagulation or flocculation, ozonation, chemical oxidation, and electrochemical treatment reported by Subki et al., [6], novel technology, such as membrane filtration and bio-sorption [7]. Among all of the possible techniques, the photocatalysis process has proven to be the most favorable technology in textile wastewater treatment, where the most organic matter can be oxidized to water, carbon dioxide, and simple inorganics materials using light radiation and selected catalysts [8].

Due to the expensive production of chemical photocatalyst compounds, utilizing waste materials as adsorbents integrated with metal oxides to form composite photocatalysts has become the main focus of researchers nowadays in maintaining the sustainability of both the treatment and the environment. Mollusk shells catch attention as a potential derived catalyst in dye removal due to the high content of calcium carbonate (CaCO3) in raw material to produce calcium oxide (CaO) as the most promising heterogeneous alkali catalyst obtained *via* the calcination process [9]. As the most heavily traded bivalve ample mollusk shell, cockle shells have been recorded to have a high percentage of CaO contained in the natural compound of the shell, which is 99.17% of CaCO3 before calcined [10] while mussel shells came in the second place with 98.37% of CaCO3 [11].

In order to get the small microstrain of CaO from CaCO3 through the calcination process, Sari et al., [12] investigated the effect of calcination temperature on the crystallization of CaO from green mussel shells. They found out that the calcined CaO at 950°C obtained a small microstrain compared to the samples calcined at other temperatures. Besides, CaO calcined at 950°C exhibited the largest crystallite size, meaning it had high crystallinity and a shortened amorphous phase. The CaO calcined at 950°C showed a small microstrain compared to the other samples, meaning the crystal defects in the sample were small.

The particle sizes of composite photocatalysts are also one of the factors for enhanced photocatalytic performances. Based on previous researchers, they used various methods for synthesizing mollusk shells with various particle sizes produced. Among cockles, scallops, oysters, pyramidella, green mussels, razor clams, golden apple snails, and snail shells, the highest CaO is produced from cockle and green mussel shells [11]. Commonly, the synthesized waste shell involves the same steps, which are cleaning, drying, crushing, grinding, sieving, and calcination process. Mostly, the particle sizes of CaO are produced in micro-size as prepared by Buasri with his group research starting 2013 [11] until 2015 [13] with various types of shells. Interesting to note that Gbadeyan et al., [14] was successfully synthesized snail waste shells in nanosized *via* dry and wet ball milling methods.

The ratio of adsorbent with metal oxides to form a composite photocatalyst needs to be considered in order to get the highest removal of pollutants. Dzinun et al. [15] found that the optimum ratio for the highest adsorption and MB photocatalytic degradation was achieved by using a (1/9) ratio of TiO2/eggshell. Therefore, in this study, 9:1 ratio of mollusk shell with TiO2 was investigated. In total 100% of mollusk and cockle shell was used as the control sample with indicated ratios of 10:0. The composite photocatalyst was prepared by solid-state dispersion (SSD) method for MB removal in the suspension system.
