Preparation Of Biomass-Derived Composite Photocatalysts For Application In Dye Degradation

Biomass-derived activated carbons（BACs）prepared through the carbonation of agricultural wastes at an elevated temperature followed by chemical activation are widely used in the field of water treatment.However,the practical application of BACs has been largely restricted due to the fact that the removal of pollutants is mostly realized by physiochemical adsorption on BACs,rather than structural degradation into non-hazardous matters.In this context,the photocatalytic oxidation technology（also known as photocatalysis）has received extensive research attention in recent years thanks to its ability to completely degrade harmful and toxic organic pollutants.Cadmium sulfide（CdS）semiconductor recently becomes a research hotspot in view of its high activity and conversion efficiency of visible light irradiation,yet the instability（agglomeration and photocorrosion）,the easy recombination of photo-generated electrons and holes,and the difficulty in recycling have limited its advances in practical applications as photocatalysts.In order to address the issues related to both BACs and CdS,this project focuses on using BACs as material supports for loading CdS to prepare composite photocatalysts for the removal of dye pollutants in aqueous systems.Two types of biomass（lotus seedpod and lotus leaf）were selected as precursors to prepare BACs by high—temperature carbonization and KOH activation.The CdS semiconductor was synthesized by hydrothermal method.Calcination of BACs and CdS mixtures resulted in the formation of various composite photocatalysts.The effects of biomass resources and activation temperatures on surface structures and properties of BACs were systematically studied.Moreover,mechanism of photocatalysis coupled to the interface interaction of composite photocatalysts was investigated based on photocatalytic degradation of dye pollutants under visible light irradiation.Following results were concluded:（1）The Brunauer-Emmett-Teller（BET）surface area of BACs increased with the incensement of activation temperature,and the surface structures of BACs were dominated by micro-pores.Of special notice is the BET surface area of BACs depended greatly on biomass resources,ranging from 957 to 1539 m² g^-1（lotus seedpod resource）and 1184 to 1807 m² g^-1（lotus leaf resource）,respectively.（2）The CdS semiconductor prepared via hydrothermal synthesis was in mixed hexagonal and cubic phases,which corresponded to cluster and rod-shaped morphology of CdS,respectively,as demonstrated by X-ray diffraction（XRD）and transmission electron microscopy（TEM）.The band gap energy of CdS semiconductor estimated from the diffuse reflectance spectra（DRS）was ca.2.22 eV,which might induce photocatalytic reactions under visible light irradiation.（3）The BET surface area and average pore size of composite materials were generally lower than that of the corresponding BACs,while rationally high BET surface area was maintained to display fairly good adsorption capacity.Furthermore,the morphology and crystal phases of CdS remained unchanged after calcinations into composite photocatalysts with enhanced crystallinity.（4）In comparison with the individual CdS semiconductor,the composite photocatalysts exhibited higher photocatalytic efficiency and stability,as well as excellent recyclability.Among them,CdS@LAC-800 showed the best photocatalytic performance.The photocatalytic degradation of rhodamine B（RhB）shown by CdS@LAC-800 reached ca.96%within 60 min,which was more than 50 times that of the CdS precursors.Recycling experiments suggested that CdS@LAC-800 was able to maintain rationally high photocatalytic activity after four consecutive cycles,and the morphology and crystallinity of CdS in composite photocatalysts remained nearly unchanged,indicating outstanding recyclability and chemical stability of the composite photocatalysts.（5）The mechanism of photocatalytic reaction was investigated by means of various characterization methods and illustrated as follows:The CdS on composite photocatalyst was first excited by visible light to produce photo-generated electrons and holes,and the photo-generated electrons were transported to the surface of BACs supports to reduce oxygen into O₂·^-radicals.O₂·^-radicals and holes further participated in oxidation of H20 and OH-into ·OH radicals.Three active spicies（O₂·^-,·OH,and h⁺）were determined as the major active species and participated in photocatalytic degradation of dye pollutants under visible light,following an order of O₂·^->·OH>h⁺.Overall,this study have revealed that the composite photocatalysts display excellent and integrated adsorption-photocatalysis property.Further to their noticeable chemical stability and recyclability,These materials have promised great potentials in water treatment and other environmental-related fields.