Harvesting and preparation of sludge-based fibrous adsorbent for heavy metals removal: Effects of organic substances molecular weight

The proper recycling of anaerobically digested sludge (ADS) is a great challenge, and the recovering and har-vesting of the dissolved organic substances (DOS) in ADS to prepare innovative adsorbents may be an attractive option. This study uses ADS as the raw material to develope the DOS based amyloid-fibrils-like adsorbent (DOS-AFs) for heavy metal removal, and the mechanisms involved in DOS fibrosis and heavy metals adsorption are illustrated. Results indicate that the molecular weight (MW) of the DOS precursors greatly affect the fibrosis extent and the DOS-AFs surface morphology. DOS with low MW tends to form the rigid and smooth rod-like DOS-AFs with long-chain fibrous structure, whereas the macromolecular DOS is more likely to form DOS-AFs with rougher and more complicate branching structure. The formation of beta-sheet structured DOS-AFs is largely formed from low-MW DOS rather than high-MW ones. The as-prepared DOS-AFs with different DOS fractions as pre-cursors show different affinities towards heavy metals. The adsorption sites on macromolecular DOS-AFs are more likely to bind silver ion, whereas those small molecule DOS-AFs exhibit stronger affinity towards copper ion and nickel ion. The humic acid fraction in DOS plays two different roles in terms of heavy metals adsorption. The strong ligand of humic acid within DOS-AFs favors heavy metals adsorption, and the fibrosis extent of low-MW fraction is rather weak and the formation of soluble heavy metals complexes adversely inhibits adsorption performance. This study provides an alternative to harvest organic substances in ADS and to prepare valuable adsorbents towards heavy metals, although further studies are necessary to advance it in engineering practice.