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N and Fe codoped carbon dots (N,Fe-CDs) were fabricated from citric acid, L-glutamic acid and ferric chloride via a hydrothermal method for the photocatalytic removal of S2− from kraft washing effluents (KWE). The N,Fe-CDs were fluorescent nanoparticles (average size of 3.18 nm) and catalyzed the oxidation of S2− following a first-order kinetic model with an activation energy of 33.77 kJ/mol. The N,Fe-CDs tolerated elevated temperatures as high as 80 °C without catalyst deactivation. The N,Fe-CDs catalysts were reusable for at least four cycles, preserving over 90% of the activity. In the treatment of KWE from the kraft pulping of eucalyptus, the concentration of S2− was decreased by the N,Fe-CDs from 1.19 to 0.41 mmol/L in 6 h. Consequently, near complete remediation was obtained in 24 h. In addition, half of the chemical oxygen demand was removed after treatment with 500 mg/L of the N,Fe-CDs. In addition, the present photocatalyst was safe within a concentration of 200 mg/L, as indicated by the acetylcholinesterase inhibition test. Our findings may help develop a cleaner production process for kraft brownstock washing.

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Antiscolic herbal compounds (AHCs) are fast-growing biopesticides but must be removed afterwards from harvested crops due to their potential toxicities towards animal acetylcholine esterase (AchE). In this work, we investigated the inhibitory effect of several typical AHCs on AchE as well as their detoxification by bacilli probiotics, another group of green agricultural additives. Results showed the AchE inhibition activity of studied AHCs follows an order of berberine (IC50 = 12.16 μmol/L) > osthole (233.21 μmol/L) ≈ matrine (262.41 μmol/L) >> trigonelline (no inhibition). Molecular simulation predicted that berberine, matrine and osthole combine with AchE preferentially in the catalytic channel for acetylcholine (Trp84 and His440), but forming more stable complexes. At the IC50 dosage, the addition of Bacillus subtilis (106~107 CFU/mL) or several other bacilli species detoxified osthole from 50% AchE inhibition to around 30%. Slight detoxification of matrine was also observed but the toxicity of berberine was not influenced after incubated with bacilli probiotics. Conversely, the three AHCs could completely inhibited the growth of B. subtilis following an order of berberine (0.15 mmol/L) > Matrine (9 mmol/) > osthole (1.5 mmol/L). B. subtilis Ruizhen@ exhibited stronger tolerance to AHCs than other commercial strains. Among other tested species, B. mucilaginosus was less sensitive than B. subtilis while B. laterosporus and P. polymyxa were resistant to AHCs. The potential antagonistic actions between AHCs and bacilli probiotics maybe useful in removal of residual herbal toxicity with green bacterial sweeper.