Key roles of the crystal structures of MgO-biochar nanocomposites for enhancing phosphate adsorption.

The affinity of biochar (BC) adsorbing phosphate was weak, while generation of magnesium oxide (MgO)-BC nanocomposites that transformed the crystal structures of BC would change the adsorption processes in improving the phosphate adsorption. Hereon, four different crystal structure of absorbents were selected to illustrate why the crystal structures and surface properties of absorbents were of great importance for the phosphate adsorption. The results showed that MgO/KBC with higher combination degree between MgO and KBC could change the normal crystal structure (MgO/KBC1, MgO phase (dominant)) to C-Mg-O phase (dominant). Therefore, MgO/KBC could achieve highest adsorption rate (k2, 8.059 g mg-1 min-1) and qm (maximal adsorption capacity, 121.950 mg g-1) for phosphate adsorption among absorbents, and even it had high anti-interference capacity for anions and natural organic matter (NOM). The mechanisms of MgO/KBC for phosphate adsorption were hydrogen-bond interaction, inner-sphere complexation and surface chemical adsorption; adsorption of phosphate on MgO/KBC1 was mainly controlled by inner-sphere complexation (Mg-O-PO3H2-, Mg-O-PO3H2- species). In addition, the adsorbability of MgO/KBC for phosphate could be restored after recalcination, which further proved that an efficient nanocomposite, calcinated from waste biomass (fallen leaves), was proposed to control eutrophication.

Removal of Cu and Pb from contaminated agricultural soil using mixed chelators of fulvic acid potassium and citric acid.

This study investigated the application of a specific soil washing method to remove Cu and Pb from contaminated agricultural soil. To develop an efficient leaching agent of heavy metal compounds for use in farmland soil, a mixed chelator (MC) was prepared using potassium fulvic acid (PFA, 3.2%) and citric acid (CIT, 0.16 M) in a volume ratio of 4:1 (PFA:CIT = 4:1); the optimal solid-liquid ratio (S/L = 1:20), initial pH value (4.51) and contact time (360 min) were also explored. Under optimal conditions, the removal efficiencies of MC for Cu and Pb were 42.92% and 50.46%, respectively, both of which performed better than PFA (27.86% of Cu and 17.91% of Pb) and CIT (42.04% of Cu and 41.46% of Pb). The effective states, bioavailability and relative mobilities of Cu and Pb in soil were also efficiently reduced by MC, which also increased the stability of these elements, thereby lowering the risk to soil health. More importantly, MC not only had little effect on the soil physicochemical properties (e.g., pH, organic matter (OM), cation exchange capacity (CEC), ammonium nitrogen (AN), available phosphorus (AP) and rapidly available potassium (AK)), but also improved the restored soil. Furthermore, soil structure, surface elements and the enzyme activity did not exhibit significantly loss. Therefore, MC has great potential for remediating agricultural soil.

Enhanced adsorption of Cd(II) from aqueous solution by a magnesium oxide-rice husk biochar composite.

In this study, a magnesium oxide-rice husk biochar composite (MgO-BCR) was successfully prepared by a MgO impregnation method, and its adsorption performance was investigated in Cd(II) aqueous solution. A pseudo-second-order kinetic model described the Cd(II) adsorption behaviour on BCR and MgO-BCR well, while a Langmuir adsorption isotherm was more suitable for Cd(II) adsorption on the adsorbent. The fitting results of the monolayer model indicated that the number of ions captured by per site varied between 0.97 and 1.09. The calculated thermodynamic parameters indicated that Cd(II) adsorption onto MgO-BCR was spontaneous and endothermic. Characterisation of the adsorbent revealed that in situ precipitation, surface complexation, and electrostatic attraction contributed to the Cd(II) adsorption. The adsorption capacities of rice husk biochar (BCR) and MgO-BCR for Cd(II) reached 6.36 and 18.1 mg/g, respectively. The results demonstrated that MgO-BCR composite could be used as an effective and eco-friendly adsorbent to enhance the removal of Cd(II) from aqueous solution.