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, π-π 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: R²> 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.