Residual and ecological risk assessment of heavy metals in fly ash from co-combustion of excess sludge and coal

Abstract

Co-combustion of municipal excess sludge (ES) and coal provides an alternative method for disposing ES. The present study aims to investigate the residual and ecological risk of heavy metals in fly ash from co-combustion of ES and coal. The total concentration and speciation distribution of heavy metals, characterization of SEM, EDX, XRD and leaching test were carried out to assess the fly ash in this study. The results showed that the total concentrations of Cu, Zn and Mn were higher than others in fly ash, and most heavy metals were concentrated in fine particles. For Cd, Cr and Pb, the percentages of speciation of F4 and F5 were all over 90%, suggesting the relatively lower leaching toxicity. The leaching percent of all heavy metals was lower than 5% by two diluted HNO3 solutions for fly ash. The potential ecological risks increased with the decrease of particle size of fly ash, and Cd accounted for the main fraction for ecological risk despite of lower concentration in comparison to other measured heavy metals.

Ecological Risk Assessment and Influencing Factors of Heavy-Metal Leaching From Coal-Based Solid Waste Fly Ash

“…As can be seen from Figure 6B, the leaching of heavy metals generally showed an upward trend with a decrease in particle size, which is consistent with the results of Zhou et al (2015a) and Tang et al (2021). Cd, Cr, Cu, and Ni leaching increased with decreasing particle size, with no significant differences observed among these metals.…”

“…Zhou et al (2015a) investigated the content and morphology of heavy metals in fly ash composed of different particle sizes, finding that volatile metals (such as Zn, Pb, Cu, and Cd) tended to be more abundant in finer particles, resulting in a higher risk to human and environmental health. Furthermore, Tang et al (2021) also found that heavy metals are more concentrated in finer particulate matter, with the potential ecological risk increasing as the particle size of fly ash decreases. Risk assessment is an important way to evaluate the environmental safety of heavy-metal risks.…”

“…Qian et al (2016) used the Nemerow index to evaluate heavy metals in soil. Furthermore, Tang et al (2021) used the ecological risk index (RI) to evaluate the risk level of heavy-metal pollution.…”

Sludge-based biochar with lanthanum modification for phosphate recovery from wastewater streams

“…The ecological risk index (RI) was applied to assess the risk based on heavy metal leaching from sludge-based biochar application based on eqn (1) and (2): 15,16 RI = ∑ E i where E i is a single potential ecological risk factor for each heavy metal including Cd, As, Cu, Ni, Zn and Pb, respectively; T i is the toxic factor of each heavy metal and the values are 30 (Cd), 10 (As), 5 (Cu), 5 (Ni), 1 (Zn) and 5 (Pb), respectively; 15,17 C i and C 0 are the measured value and control reference value for each heavy metal, respectively. The classification of risk levels based on E i and RI is shown in Table S1 of the ESI †…”

“…The ecological risk index (RI) was applied to assess the risk based on heavy metal leaching from sludge-based biochar application based on eqn (1) and (2): 15,16…”