Effects of peanut shell magnetic biochar on anaerobic digestion of pigment sludge and heavy metal speciation

The addition of exogenous additive magnetic biochar (MBC) is an effective technology to solve the low efficiency of pigment sludge anaerobic digestion and the low efficiency of heavy metal passivation. To improve the anaerobic digestion efficiency of pigment sludge and stabilize the heavy metals, peanut shells, an agricultural waste product, were prepared as biochar (BC). BC had a limited effect on anaerobic digestion and heavy metal passivation, and then was magnetically modified to obtain magnetic biochar (MBC) to enhance impact effects. Group A was set as a control group. The prepared biochar (BC) and magnetic biochar (MBC) as exogenous additives were added in groups B and C, respectively. The effects of MBC on the anaerobic digestion efficiency of pigment sludge and the passivation of heavy metals Cr and Ni were investigated. The results showed that the MBC additive could effectively enhance the production of volatile fatty acids (VFAs) during anaerobic digestion. The maximum VFAs concentration of groups A, B and C reached 603.9, 642.5 and 914.5 mg/L, respectively. The maximum VFAs concentration of group C was enhanced by 51.4% compared with group A. In addition, the MBC additive was conductive to maintain the pH stability of the digestion system and provided favorable environmental conditions to enhance the anaerobic digestion efficiency. The VS removal rate and cumulative methane production were 45.5%, 48.3%, 62.7% and 10.01, 10.63, 15.65 L in groups A, B and C, respectively. As such, average daily methane production and average daily methane yield obtained 333.6, 354.2, 521.6 mL and 116.5, 121.6, 159.8 mL/g in groups A, B and C. Clearly, the optimal anaerobic digestion efficiency was achieved in group C and the worst in group A. It benefits from both biochar and Fe3O4 contributing to the Direct Interspecies Electron Transfer (DIET) among microorganisms. MBC can facilitate the coupling of dual direct interspecies electron transfer effect of biochar and Fe3O4. The efficiency of electron transfers from the donor to the acceptor of the anaerobic digestion system was significantly enhanced, thus increasing the rate of organic matter consumption and methane production. The heavy metals (Cr and Ni) contained in pigment sludge can also cause significant secondary contamination besides biorefractory organics. Although the total concentration of heavy metals can be a useful indicator of digested sludge pollution, the potential toxicity of heavy metals is linked to their bioavailability and chemical speciation. The chemical speciation of heavy metals in pigment sludge consist of four types, namely exchangeable, reducible, oxidizable and residual speciation. The exchangeable speciation is the most bioavailable, the reducible speciation is the next most bioavailable, while the oxidizable and residual speciation exist in complexes and can be stable in the natural environment or in the sludge for a long time. The findings suggest that MBC additives significantly affected the chemical speciation of heavy metals Cr and Ni, which contributed to reducing the bioavailability of heavy metals (BHM) in digested pigment sludge. Compared with the feedstock, the mass percentages of exchangeable speciation Cr decreased by 7.8%, 13.0% and 43.6% in groups A, B and C, respectively. As for reducible speciation Cr, reduced by 8.9%, 35.0%, 61.6%. While the mass percent concentration of exchangeable speciation Ni got a 7.1%, 18.5% and 41.0% reduction in groups A, B and C, respectively. As for reducible speciation Ni, got 9.9%, 22.3% and 59.2%. In groups A, B and C, the passivation efficiency of Cr and Ni achieved 8.2%, 20.9%, 50.1% and 8.4%, 18.5%, 48.9%, respectively. The above results indicate that MBC could effectively improve the anaerobic digestion efficiency of pigment sludge and decrease the bioavailability of heavy metals (Cr and Ni). It contributes to the stabilization, innocuity treatment and resource utilization of pigment sludge. © 2022 Chinese Society of Agricultural Engineering. All rights reserved.