Environmental applications of activated carbons: Synthesis and applications in adsorption of organic contaminants

The use of lignocellulosic wastes as precursors of carbon materials has become an interesting alternative in the adsorption of organic compounds, due to their resulting excellent textural and chemical properties, promoting the binding interactions between the adsorbate and the surface activated carbon. Considering that the structural differences of the precursors play an important role in the creation of the porosity of the carbon, it is crucial to select an adequate precursor. It has been reported that sawdust, peach stones, olive stones, rice husks, fruit pulps, plant wastes, etc., which are mainly composed of cellulose and lignin, are suitable precursors for the synthesis of activated carbons via chemical activation. Additionally, earlier studies have shown that chemical activation by impregnation with phosphoric acid at moderate temperatures leads to activated carbons with high surface area and a medium-high degree of mesoporosity. These materials appear as good adsorbents because of their excellent textural and physico-chemical properties. It is known that the surface functionalization can change the reactivity and selectivity of a carbon surface, which plays an important role in the adsorption. The application in adsorption of these activated carbons was studied. Adsorption, since it is a non-destructive tertiary process, has been revealed as one of the most advantageous physico-chemical techniques in wastewater treatment processes. In the research practice, it is very important to find a material showing high binding affinity toward the contaminant, and high selectivity and adsorption capacity properties. Commercial activated carbons possess most of these features, since they show a desirable affinity to hundreds of organic and inorganic compounds. Since the pore diffusion seems to be the controlling mechanism in activated carbon adsorption, the use of commercial adsorbents is usually associated with high expensive and environmental undesirable regeneration operations. In this chapter, lignocellulosic-derived activated carbons obtained from peach stones and rice husk, showing mesoporous and macroporous properties, have been synthesized. These activated carbons have been tested in the removal of organic contaminants (as pharmaceutical, dyes, etc.) from water. The aim of the chapter is to compare the behaviour of the adsorbents, regarding the adsorption capacity as well as the kinetic performance, through the development of equilibrium and dynamic adsorption tests. Additionally, desorption tests using different eluents will be accomplished, in order to determine the recovery efficiencies for the contaminants.