**Ainoa Murcia-Salvador, José A. Pellicer, María Isabel Rodríguez-López, Vicente Manuel Gómez-López, Estrella Núñez-Delicado and José A. Gabaldón \***

Dpto. de Ciencias de la Salud., Universidad Católica San Antonio de Murcia (UCAM), Avenida de los Jerónimos s/n, 30107 Murcia, Spain; amurcia6@alu.ucam.edu (A.M.-S.); japellicer@ucam.edu (J.A.P.); mirodriguez@ucam.edu (M.I.R.-L.); vmgomez@ucam.edu (V.M.G.-L.); enunez@ucam.edu (E.N.-D.) **\*** Correspondence: jagabaldon@ucam.edu; Tel.: +34-968-278-622

Received: 18 February 2020; Accepted: 8 March 2020; Published: 11 March 2020

**Abstract:** Eggshell, a waste material from food manufacturing, can be used as a potential ecofriendly adsorbent for the elimination of textile dyes from water solutions. The adsorption process was evaluated varying factors such as initial dye load, contact time, pH, quantity of adsorbent, and temperature. The initial dye load (Direct Blue 78) was in the range of 25–300 mg/L. The kinetics of adsorption were analyzed using different models, such as pseudo-first-order, pseudo-second-order, and intraparticle diffusion model. Also, the experimental data at equilibrium were studied using Freundlich, Langmuir, and Temkin isotherms. The kinetics followed pseudo-second-order, then pseudo-first-order, and finally the model of intraparticle diffusion. The results obtained for data at equilibrium follow the order: Freundlich > Langmuir > Temkin. The adsorption equilibrium showed a maximum capacity of adsorption (qmax) of 13 mg/g at pH 5, and using 0.5 g of eggshell. Dye adsorption was enhanced with increasing temperatures. The thermodynamic study revealed the spontaneity and endothermic nature of the adsorption process. The desorption study shows that the eggshell could be reused in different adsorption/desorption cycles. A novel advanced oxidation process could degrade more than 95% of the dye. The results show that eggshell is a waste material useful to remove hazardous dyes from wastewater, which may alleviate the environmental impact of dyeing industries.

**Keywords:** adsorption; eggshell; Direct Blue 78; kinetics; isotherms; pulsed light
