A Systematic Analysis and Review of the Fundamental Acid-Base Properties of Biosorbents

Abstract

[Abstract] A broad variety of materials of biological origin have been successfully used in recent decades for the removal of pollutants from solution. These biosorbents present a range of natural polymers that play a key role on their adsorption capacity. It is therefore critical to understand the physicochemical properties of the chemical groups that form these polymers. According to bibliography, less than 3% of biosorption papers include studies on proton binding. The acid-base properties of biomass are affected by pH, ionic strength and medium composition. Nevertheless, these crucial parameters are not always considered during biosorption studies. This review outlines the major advances on proton binding data interpretation and modelling on biosorbents. In addition, we propose some experimental considerations that cover all issues raised in this review concerning the acid-base properties of biosorbents. Only 30% of the reviewed papers that study algae, agricultural wastes or lignocellulosic materials use Donnan or double-layer surface models to account for electrostatic interactions on proton binding. Expressions for activity coefficients, such as Debye-Hückel or Pitzer equations, are shown only in c.a. 15% of these papers. Moreover, studies investigating a range of ionic strengths represent a 40%, while this variable is not even considered in 20% of the papers. We could not find any biosorption study related to specific salt or Hofmeister effects. Moreover, in 6 out of 10 papers there is important experimental information missing such as the calibration of the electrodes. We consider therefore that there is an important need for reviewing the role of proton binding on biosorption studies.