Decolorization of triarylmethane dyes, malachite green, and crystal violet, by sewage sludge biochar: Isotherm, kinetics, and adsorption mechanism comparison

Sewage sludge biochar (SBC) was used as adsorbent to study the adsorption behavior of triarylmethane dyes, malachite green (MG; diaminotriphenylmethane), and crystal violet (CV; triaminotriphenylmethane). SBC exhibited high content (g/kg) of Al (65.8), P (64.6), Ca (57.3), and Fe (44.6). The Langmuir model showed that the affinity of MG for the surface of SBC was 22.6-times that of CV's (K-L=0.0053l/mg); maximum Langmuir monolayer adsorption capacity of 69.5 mg/g for MG and 49.0 mg/g for CV. Similar functional groups and adsorption mechanisms like hydrogen bonding, pi-pi interaction, electrostatic interactions, and ion exchanges governed both MG and CV adsorption onto SBC. Both physisorption and chemisorption were involved in both dyes' adsorption (Redlich-Peterson model: R-2> 0.900) Leachability tests showed a dependency of leached metallic ions on the type of dye employed, where ion exchange was dominated by P, Al, Ca, K for MG, and Na, K, Ca for CV. Interestingly, although minimal, the standalone contribution of biochar-free ions on MG and CV decolorization was, respectively, 13% and 7.7% (Fe), 6.7% and 2.3% (K), 2.9% and 0% (Ca), and 0% and 0.8% (Mg), which showed that some adsorption-unrelated mechanism may have also contributed to decolorization of CV and MG.

Automated summary

Sewage sludge biochar exhibited high metal content and strong adsorption capacity for malachite green and crystal violet dyes. The adsorption mechanisms for both dyes include hydrogen bonding, pi-pi interaction, electrostatic interactions, and ion exchanges.