Optimization of Lead and Diclofenac Removal from Aqueous Media Using a Composite Sorbent of Silica Core and Polyelectrolyte Coacervate Shell

The modification of inorganic surfaces with weak cationic polyelectrolytes by direct deposition through precipitation is a fast approach to generating composites with high numbers of functional groups. The core/shell composites present very good sorption capacity for heavy metal ions and negatively charged organic molecules from aqueous media. The sorbed amount of lead ions, used as a model for priority pollutants such as heavy metals, and diclofenac sodium salt, as an organic contaminant model for emerging pollutants, depended strongly on the organic content of the composite and less on the nature of contaminants, due to the different retention mechanisms (complexation vs. electrostatics/hydrophobics). Two experimental approaches were considered: (i) simultaneous adsorption of the two pollutants from a binary mixture and (ii) the sequential retention of each pollutant from monocomponent solutions. The simultaneous adsorption also considered process optimization by using the central composite design methodology to study the univariate effects of contact time and initial solution acidity with the purpose of enabling further practical applications in water/wastewater treatment. Sorbent regeneration after multiple sorption-desorption cycles was also investigated to assess its feasibility. Based on different non-linear regressions, the fitting of four isotherms (Langmuir, Freundlich, Hill, and Redlich-Peterson models) and three kinetics models (pseudo-first order (PFO), pseudo-second order (PSO), and two-compartment first order (TC)) has been carried out. The best agreement with experiments was found for the Langmuir isotherm and the PFO kinetic model. Silica/polyelectrolytes with a high number of functional groups may be considered efficient and versatile sorbents that can be used in wastewater treatment processes.

Automated summary

The direct deposition of weak cationic polyelectrolytes on inorganic surfaces through precipitation is a fast method for producing composites with numerous functional groups. These core/shell composites exhibit excellent sorption capacity for heavy metal ions and negatively charged organic molecules in aqueous media. The adsorption of lead ions and diclofenac sodium salt depends on the organic content of the composite and is less affected by the nature of contaminants due to different retention mechanisms. Two experimental approaches, simultaneous adsorption from a binary mixture and sequential retention from monocomponent solutions, were investigated. Additionally, process optimization using the central composite design methodology was carried out to study the effects of contact time and initial solution acidity. The suitability of silica/polyelectrolytes as efficient sorbents for wastewater treatment was demonstrated through sorbent regeneration and the fitting of isotherm and kinetics models.