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Marine shell wastes were thermally activated and characterized as aragonite and calcite phases and were used in the removal of synthetic anionic dyes, Bright Blue Acid (NB180) and Reactive Red 133 (RR133). Benefited marine shells were classified as low-cost (USD 0.33/g of adsorbent) in comparison with other reported materials. Furthermore, the absence of chemicals in the adsorbent preparation allows its further employment in economic activities. The coexistence of adsorption and exchange-precipitation reaction was responsible for up to 93% of dye removal, whilst the maximum adsorption capacities were 225 mg g -1 for NB180 and 36 mg g -1 for RR133. The observed kinetic behavior of the dye removal by the adsorbent allowed the proposal of a mechanism for dye-adsorbent interaction in liquid-solid interface considering both adsorption and exchange-precipitation reaction. Contribution of the exchange-precipitation reaction in the removal process was quantified as being approximately 75% for NB180 and 25% for RR133. The mathematical model that phenomenologically described the kinetic behavior of the dye removals gave the magnitude order of the kinetic parameters as k ads  = 8.67-9.49 min -1 and k p  = 1.18-2.84 min -1 , due to the adsorption and the (exchange-reaction)-precipitation, respectively. This work indicates the step (exchange reaction)-precipitation as an additional contribution to improve the dye removal from aqueous effluents, achieving in the evolution of the process up to 24% in terms of kinetic selectivity of removal., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Malacoculture waste (Anomalocardia brasiliana) shellfish shells (ABSS) were evaluated as adsorbents of Nylosan Brilliant Blue (NBB) acid dye. The ABSS were thermally activated at 1,000 °C for 10 h and then characterized by Fourier-transform infrared spectroscopy, analysis of specific surface area (BET), X-ray diffraction (XRD), and scanning electron microscopy. Point of zero charge (PZC) analysis of ABSS verified pH PZC 13.0. The study of kinetics showed that the pseudo-second-order model fit the experimental data best and the system reached equilibrium within 5 min. Adsorption isotherms followed the Langmuir-Freundlich isotherm and ABSS reached an outstanding maximum adsorption capacity of 405 mg·g -1 under the following optimum conditions: pH 12.4, 303 K, 450 rpm, 2.0 g of adsorbent, and 150 μm average particle size. These conditions were obtained after a previous statistical analysis of the variables. Enthalpy and Gibbs energy obtained in the thermodynamics experiments were -23.79 kJ·mol -1 and -4.07 kJ·mol -1 , respectively. These parameters confirm that the process is exothermic, spontaneous, and indicative of the physical nature of the adsorption. The adsorption of NBB onto ABSS tended to be more favorable at a lower temperature. Low value of enthalpy suggested that weak binding forces, such as electrostatic interactions, govern the sorption mechanism. ABSS high availability in the environment, its low toxicity and high efficiency make it a promising ecofriendly adsorbent of textile dyes.