Response surface optimization for cadmium biosorption onto the pre-treated biomass of red algae Digenia simplex as a sustainable indigenous biosorbent

Abstract

Background: Cadmium pollution from industrial effluent can cause major health concerns, so it must be removed from wastewater prior to disposal. The objective of this study was to remove cadmium (Cd²⁺) from aquatic environments using red macroalgae Digenia simplex pretreated with calcium chloride (CaCl₂) (DSC).

Methods: Batch adsorption studies were carried out to evaluate the individual impacts of adsorbent-metal contact time, cadmium concentration, and temperature on the cadmium removal efficiency and biosorption capacity. The Box-Benhken experimental design of response surface methodology was also used to investigate the relationship between different factors (pH, Cd²⁺ concentration and algal dose) and the cadmium removal efficiency of pretreated D. simplex.

Results: The highest removal efficiency of 97.27% was achieved by combining different optimal parameters, including pH 5.78, initial Cd²⁺ concentration of 24.79 mg/L, and adsorbent dosage of 6.13 g/L. Moreover, cadmium removal from agricultural wastewater samples by pretreated D. simplex was evaluated under the optimal conditions, and the removal rate excessed 97%. Kinetic and isotherm investigations showed that the pseudo-second-order, Freundlich, Langmuir, and Dubinin-Radushkevich models of cadmium biosorption on pretreated algal biomass correlated well with the experimental biosorption data, implying that the biosorption of Cd²⁺ is a homogeneous monolayer and multilayer chemisorption process. The equilibrium isotherm data indicated that the biosorption capacity of the biosorbent was 11.16 mg/g as determined by the Langmuir model. Furthermore, the biosorption process was evaluated as an endothermic process with entropy and enthalpy values of 0.134 kJ/mol K and 38.01 kJ/mol, respectively. The functional groups, surface morphology, and elemental composition of the algal biomass were investigated, revealing the porous nature of the cell surface and the abundance of functional groups responsible for the Cd²⁺ biosorption process. These results suggest that DSC biomass can be used as a biosorbent for the effective removal of Cd²⁺ ions from effluent due to its availability and strong biosorption capability.

Keywords: Biosorption; Cadmium ion; Calcium chloride; Digenia simplex; Optimization; Response surface methodology.

Figure 1. FTIR of pretreated Digenia biomass (A) prior and (B) after cadmium biosorption.

Figure 2. Scanning electron micrographs of pretreated Digenia biomass (A) prior and (B) after Cd²⁺ ions biosorption.

Figure 3. EDX spectra of pretreated Digenia biomass (A) prior and (B) after cadmium biosorption.

Figure 4. Impact of (A) contact time, (B) initial cadmium concentration, and (C) temperature on the removal efficiency (%), and biosorption capacity of Cd²⁺ ions (q_e) (biosorbent dose = 5 g/L, initial Cd²⁺ concentration = 30.

Figure 5. (A) Internally studentized residual versus expected response plot, (B) expected removal efficiency against the actual efficiency plot, and (C) number of run for Cd²⁺ ion.

Figure 6. The three-dimensional (left) and contour (right) graphs of Cd removal efficiency by pretreated D. simplex as a function of (A) pH and initial metal concentration, (B) initial Cd concentration and algal dose, (C) pH and algal dose.

Figure 7. Desirability ramp for the optimization of the independent factors and cadmium removal efficiency by pretreated Digenia biomass.