Removal of toxic elements from aqueous environments using nano zero-valent iron-and iron oxide-modified biochar:a review

Biochar(BC)has gained attention for removal of toxic elements(TEs)from aqueous media;however,pristine biochar often exhibits low adsorption capability.Thus,various modification strategies in BC have been developed to improve its removal capability against TEs.Nanoscale zero-valent iron(nZVI)and iron oxides(FeOx)have been used as sorbents for TE removal.However,these materials are prone to agglomeration and also expensive,which make their usage limited for large-scale applications.The nZVI technical demerits could be resolved by the development of BC-based composite sorbents through the loading of nZVI or FeOx onto BC surface.Nano zero-valent iron modified BC(nZVIBC),FeOx-modified BC(FeOxBC)have attracted attention for their capability in removing pollutants from the aqueous phases.Nonetheless,a potential use of nZVIBC and FeOxBC for TE removal from aqueous environments has not been well-realized or reviewed.As such,this article reviews:(i)the preparation and characterization of nZVIBC and FeOxBC;(ii)the capacity of nZVIBC and FeOxBC for TE retention in line with their physicochemical properties,and(iii)TE removal mechanisms by nZVIBC and FeOxBC.Adopting nZVI and FeOx in BC increases its sporptive capability of TEs due to surface modifications in morphology,functional groups,and elemental composition.The combined effects of BC and nZVI,FeOx or Fe salts on the sorption of TEs are complex because they are very specific to TEs.This review identified significant opportunities for research and technol-ogy advancement of nZVIBC and FeOxBC as novel and effective sorbents for the remediation of TEs contaminated water.

Use of sewage sludge biochar as a catalyst in production of biodiesel through thermally induced transesterification

Sewage sludge(SS)is a residual/semi-solid material produced from industrial and municipal wastewater treatment processes.SS contains a high content of lipids and earth alkaline metals that can be used as catalysts for various chemical applications;however,its valorization has rarely been the focus of research.This study demonstrates that SS could be a promising raw material for biodiesel production and a biochar catalyst to promote the reaction kinetics of alkylation.Thermally induced transesterification of the SS extract(SSE)was performed in comparison with the conventional homogeneous reaction.SS biochar was fabricated via pyrolysis.The highest yield(33.5 wt.%per SSE)of biodiesel production was achieved in 1 min of reaction at 305℃via thermally induced transesterification in the presence of SS biochar,while the yield of biodiesel from(trans)esterification with 5 wt.%H_(2)SO_(4)was less than 1%even after 24 h.The reaction kinetics(<1 min)of thermally induced transesterification was extraordinarily faster than that of conventional transesterification(3-24 h).The porous structure and high content of alkaline species in the SS biochar expedited the reaction kinetics.Consequently,the integrated/hybridized process for thermally induced transesterification and pyrolysis of the solid residue of SS was experimentally proved for the valorization of SS in this study.Considering that SS is being disposed of as a waste material and generates toxic chemicals in the environment,its valorization into value-added biodiesel and a catalyst could be an environmentally benign and sustainable technique.