Investigations of Cr(VI) removal by millet bran biochar modified with inorganic compounds: Momentous role of additional lactate.

In this study, millet bran biochars modified with inorganic compounds (H3PO4: P-BC, NaOH: Na-BC and K2CO3: K-BC) were prepared and applied for Cr(VI) removal to evaluate the effects of modification on biochars' physicochemical properties. The results showed that Cr(VI) reduction capacity complied with the order of Na-BC > BC > P-BC > K-BC, and reductive groups such as -OH and -NH2 played considerable roles in electrons donating. Based on this, lactate was added for further investigation of electrons transferring. The results displayed that Cr(VI) removal of all biochars was enhanced tremendously and modified biochars exhibited better Cr(VI) reduction. This may be due to the bridging effect of lactate, which could not only chelate with Cr(VI) via -COOH (or -OH) but also form hydrogen bonds with oxygen or nitrogen containing groups on biochars through the other groups, thus facilitating electrons transferring between biochars and Cr(VI). This work provided an insight into evaluation of the influence of inorganic compounds modification on both electrons donating capability of biochars and electrons transferring potential of biochars combined with lactate in Cr(VI) removal.

Degradation of anthraquinone dye reactive blue 19 using persulfate activated with Fe/Mn modified biochar: Radical/non-radical mechanisms and fixed-bed reactor study.

In this study, a heterogeneous activator was prepared via the Fe/Mn modification of sludge-derived biochar (Fe/MnBC) to achieve high-efficiency activation of persulfate (PS) for reactive blue 19 (RB19) degradation. The morphologies and chemical states of Fe/MnBC were examined by various characterizations. A comprehensive assessment was conducted to reveal the effects of biochar preparation conditions and system reaction conditions. According to the results of scavenger quenching experiments and electron paramagnetic resonance (EPR) testing, the mechanisms of Fe/MnBC combined PS system on RB19 degradation were proposed, including radical and non-radical mechanisms. The formation and involvement of sulfate radical (SO4·-), hydroxyl radical (OH·), and singlet oxygen (1O2) were proved in this system, and Fe(IV)/Mn(VII) was also speculated to participate in the non-radical degradation process. These findings give a new insight into the mechanisms of PS activated by metal-biochar composite. Besides, fixed-bed reactor (FBR) experiments indicated that the Fe/MnBC has considerable PS activation potential for dyes removal. The degradation process was further modeled by the central composite design (CCD-RSM) and artificial neural networks (ANN) methods. The statistical metrics and prediction indicated that the prediction results of ANN model were better than CCD-RSM model, and the ANN model could perfectly predict the reaction process of Fe/MnBC FBR for engineering applications.

Removal mechanisms of Cr(VI) and Cr(III) by biochar supported nanosized zero-valent iron: Synergy of adsorption, reduction and transformation.

In this study, sludge-derived biochar was prepared and utilized to support nano-zero-valent iron (NZVI-SDBC) for removing Cr(VI) and Cr(III) from aqueous solution with the aim of investigating their removal and transformation. Under the conditions of initial pH of 4, dosage of 1 g/L, temperature of 25 °C, and rotational speed of 160 rpm, 64.13% Cr species could be removed by NZVI-SDBC from Cr(VI) solution and 28.89% from Cr(III) solution. Coexisting ions experiments showed that Cu(II) and humic acids dramatically affected the removal of Cr(VI) and Cr(III), while the effect of Na(I) and Ca(II) was almost negligible. Based on this, through the coexistence and pre-loaded Cr(III) experiments, the conversion from Cr(VI) to Cr(III) was demonstrated to enhance the further attraction on Cr(VI) and promote the subsequent removal of Cr(VI). The SDBC of NZVI-SDBC could serve as electron shuttle mediator to facilitate the electron transfer between adsorbed Cr(VI) and NZVI for ortho-reduction. The transformation and removal mechanisms were further discussed by various characterizations. The kinetics of Cr(VI) removal suggested that the removal process of Cr(VI) could be divided into three phases dominated by different mechanisms (adsorption, direct/ortho reduction, electrostatic attraction), in which Cr(VI) and Cr(III) showed different behaviors of interaction. The removal of Cr(III) mainly depended on sufficient adsorption sites and the direct complexation with Fe(II). Finally, the reusability of NZVI-SDBC was assessed by adsorption/desorption recycling test. These results provided new insights into the removal and transformation mechanisms of Cr(VI) and Cr(III) by biochar-based nanocomposites.