Co-pyrolysis of sewage sludge and Ca(H2PO4)2: heavy metal stabilization, mechanism, and toxic leaching

Co-pyrolysis of sewage sludge and biomass for stabilizing heavy metals and reducing biochar toxicity: A review

“…In contrast, Gu and co-workers studied the effects of co-pyrolyzing sewage sludge with different mixtures including 0, 15, and 30 wt.% of Ca(H 2 PO 4 ) 2 on immobilizing heavy metals and reducing metals leachability. The optimal blending ratio of Ca(H 2 PO 4 ) 2 is 15 wt%, which produced biochar with more stable heavy metals and reduced the environmental risks from 65.73 to 4.39 (Gu et al 2022). The potential mechanisms for heavy metals stabilization include the formation of heavy metal chelating agents, e.g., metaphophates and polyphosphates, via condensation dehydration reactions of Ca(H 2 PO 4 ) 2 , the formation of highly aromatic metallic compounds, and physical adsorption (Fig.…”

“…4). Equations 1, 2, 3 ,4, 5 show the possible reaction pathways for the formation of zinc pyrophosphate (Zn 2 P 2 O 7 ), which is a stable form of zinc and insoluble in water (Gu et al 2022). Phosphates can also help the formation of other stables forms of lead (Pb 2 P 2 O 7 ) and pyromorphite-like minerals (Mohamed et al 2017;Gu et al 2022).…”

“…Equations 1, 2, 3 ,4, 5 show the possible reaction pathways for the formation of zinc pyrophosphate (Zn 2 P 2 O 7 ), which is a stable form of zinc and insoluble in water (Gu et al 2022). Phosphates can also help the formation of other stables forms of lead (Pb 2 P 2 O 7 ) and pyromorphite-like minerals (Mohamed et al 2017;Gu et al 2022). X-ray diffraction (XRD) analysis confirmed the formation of Pb 2 P 2 O 7 , Fe 2 Pb 3 (PO 4 ) 4 , Cd(PO 3 ) 2 , (Cu, Zn)PbVO 4 (OH) (Mottramite), and Cu(OH) 3 PO 4 after co-pyrolyzing sewage sludge with Ca(H 2 PO 4 ) 2 (Gu et al 2022).…”

“…Phosphates can also help the formation of other stables forms of lead (Pb 2 P 2 O 7 ) and pyromorphite-like minerals (Mohamed et al 2017;Gu et al 2022). X-ray diffraction (XRD) analysis confirmed the formation of Pb 2 P 2 O 7 , Fe 2 Pb 3 (PO 4 ) 4 , Cd(PO 3 ) 2 , (Cu, Zn)PbVO 4 (OH) (Mottramite), and Cu(OH) 3 PO 4 after co-pyrolyzing sewage sludge with Ca(H 2 PO 4 ) 2 (Gu et al 2022).…”

“…Ca(H 2 PO 4 ) 2 also increased the surface porosity of biochar, which can immobilize heavy metals inside the internal pores of biochar via physical adsorption. Reprinted from reference (Gu et al 2022) with permission from Elsevier Min and co-workers studied the effect of using ZnCl 2 for heavy metals stabilization in biochars obtained from camellia oleifera shells and sewage sludge co-pyrolysis. ZnCl 2 impregnation reduced the overall heavy metal content through chlorination and formation of CdCl 2 and PbCl 2 , whereas the heavy metals oxidizable fraction (F3) increased remarkably, with the most noticeable effects found in nickel (Ni) and copper (Cu).…”