The Migration and Transformation of Heavy Metals in Sewage Sludge during Hydrothermal Carbonization Combined with Combustion

Abstract

The migration and transformation behaviors of heavy metals (HMs), including Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb, during the hydrothermal carbonization (HTC) of sewage sludge (SS) were investigated. The immobilization of HMs during the combustion of solid residual (SR) produced from HTC of SS was also analyzed. With increasing HTC temperature and residence time, the majority of HMs (except As) accumulated in the SR. The residual rate of As in the SR decreased from 73.95% to 56.74% when the residence time was increased from 1h to 3h and reduced significantly from 73.95% to 37.48% when the temperature increased from 220°C to 280°C, implying that numerous arsenic compounds dissolved into liquid phase products. Although the HTC process has a positive influence on the transformation of HMs from weakly bound fractions to the more stable fractions, the exchangeable and reducible fractions of Mn, Zn, As, and Cd in the SR were still high. In addition, the leached amounts of Zn and As were high (14.61 and 6.16 mg/kg, respectively) and showed a high leaching risk to the environment. An increase in HTC temperature and residence time led to an increase of the residual rate of HMs in the combustion residual of SR, implying that the HTC process promotes the stabilization of HMs in the combustion process.

Efficient Removal of Heavy Metals from Contaminated Sunflower Straw by an Acid-Assisted Hydrothermal Process

“…In contrast, the species of heavy metals converted from unstable to stable with the increase in temperature. Numerous studies demonstrated that the acid-soluble/exchangeable and reducible fraction of heavy metals could transform into oxidizable and residual fractions through complexation during hydrothermal conversion [21,[40][41][42][43]. A possible explanation for the stabilization of heavy metals during HTC is that the unstable adsorbed or dissolved heavy metals (F1 + F2) were entrapped by the organic matter with a complex structure to form stable phases and fixation in the hydrochars, and the higher the HTC temperature, the stronger the effect.…”

Sewage Sludge Treatment by Hydrothermal Carbonization: Feasibility Study for Sustainable Nutrient Recovery and Fuel Production

“…Elemental analysis confirmed that most HM stayed in the hydrochar. An explanation for the high selectivity is the sorption property of the char [22] and the reduction of HM solubility after the HTC-treatment [40].…”

Transformation and Mobility of Cu, Zn, and Cr in Sewage Sludge during Anaerobic Digestion with Pre- or Interstage Hydrothermal Treatment

“…Similarly, HT alone can significantly affect metal speciation in sewage sludge. , Our recent study revealed that the transformation of Cu and Zn in sewage sludge during HT was highly dependent on treatment temperature . Cr exists predominantly as an organo-Cr­(III) complex in activated sludge, , and HT can convert bioavailable Cr fractions into more stable fractions , as oxidizable and residual fractions in HT hydrochars. , A recent study reported that HT under alkaline conditions favors the immobilization of Cr, while it is the opposite under acidic conditions . To the best of our knowledge, no study has systematically investigated Cu, Zn, and Cr speciation evolution in sewage sludge during combined HT/AD.…”

“…15,20 Our recent study revealed that the transformation of Cu and Zn in sewage sludge during HT was highly dependent on treatment temperature. 15 Cr exists predominantly as an organo-Cr(III) complex in activated sludge, 21,22 and HT can convert bioavailable Cr fractions into more stable fractions 23,24 as oxidizable and residual fractions in HT hydrochars. 20,25 A recent study reported that HT under alkaline conditions favors the immobilization of Cr, while it is the opposite under acidic conditions.…”