How different is the remediation effect of biochar for cadmium contaminated soil in various cropping systems? A global meta-analysis.

Cadmium (Cd) poses great threats to human health as a major contaminant in agricultural soil. Biochar shows great potential in the remediation of agricultural soil. However, it remains unclear whether the remediation effect of biochar on Cd pollution is affected by various cropping systems. Here, this study used 2007 paired observations from 227 peer-reviewed articles and employed hierarchical meta-analysis to investigate the response of three types of cropping systems to the remediation of Cd pollution by using biochar. As a result, biochar application significantly reduced the Cd content in soil, plant roots and edible parts of various cropping systems. The decrease in Cd level ranged from 24.9% to 45.0%. The feedstock, application rate, and pH of biochar as well as soil pH and cation exchange capacity were dominant factors for Cd remediation effect of biochar, and their relative importance all exceeded 37.4%. Lignocellulosic and herbal biochar were found to be suitable for all cropping systems, while the effects of manure, wood and biomass biochar were limited in cereal cropping systems. Furthermore, biochar exhibited a more long-lasting remediation effect on paddy soils than on dryland. This study provides new insights into the sustainable agricultural management of typical cropping systems.

Insights into the underlying mechanisms of stability working for As(III) removal by Fe-Mn binary oxide as a highly efficient adsorbent.

Fe-Mn binary oxide has received increasing interest in treating As(III)-containing polluted groundwater due to its low cost and environmental friendliness. Although the stability of Fe-Mn binary oxide is as important as its adsorption ability, little is known about whether and why Fe-Mn binary oxide is stable during As(III) removal. In this study, five successive cycles were conducted to evaluate the stability of Fe-Mn binary oxide for As(III) removal. As(III) oxidation/adsorption kinetics and the speciation distribution of the released Fe and Mn elements within single Fe oxide, Mn oxide, and Fe-Mn binary oxide were investigated by using characterization techniques of TEM-EDS mapping, selected area electron diffraction (SAED), and XPS combined with a binary component reactor, where Fe and Mn oxides were separated by a semipermeable membrane. The results revealed that Fe-Mn binary oxide could maintain excellent stability, although As(III) oxidation/adsorption behavior was coupled with the release of Fe and Mn ions from its surface. The great stability of Fe-Mn binary oxide for As(III) removal was attributed to the rapid return of aqueous Fe(II) and Mn(II) to the solid surface, which subsequently formed new mineral phases mediated by Fe and Mn oxides, thus considerably decreasing the loss of released Mn(II) and Fe(II).

Simultaneous introduction of K+ and Rb+ into OMS-2 tunnels as an available strategy for substantially increasing the catalytic activity for benzene elimination.

OMS-2 is one of the most promising catalysts for carcinogenic benzene elimination, and single-type alkali metals are typically introduced into the OMS-2 tunnels to modify its catalytic activity. Here, we reported a novel approach for significantly increasing the catalytic activity of OMS-2 via the simultaneous introduction of K+ and Rb+ into the tunnels. The catalytic results demonstrated that K+ and Rb+ codoped OMS-2 showed catalytic activity for benzene oxidation that exceeded those of K+ and Rb+ single-doped OMS-2, as evidenced by enormous decreases (△T50 = 106 °C and △T90 > 132 °C) in catalytic temperatures T50 and T90 (which correspond to benzene conversion percentages of 50% and 90%, respectively). The origin of the effect of K+ and Rb+ codoping on the catalytic activity of OMS-2 was experimentally and theoretically investigated via 18O2 isotope labeling, CO temperature-programmed reduction, and density functional theory calculation. The higher catalytic activity of K+ and Rb+ codoped OMS-2 was attributed to its higher lattice oxygen activity as well as its higher oxygen vacancy defect concentrations compared to the single-doped OMS-2 cases.

Enhanced catalytic activity of OMS-2 for carcinogenic benzene elimination by tuning Sr2+ contents in the tunnels.

Cryptomelane-type manganese oxides (OMS-2) have been intensively investigated for application in the catalytic oxidation of carcinogenic benzene, and doping metal ions in the OMS-2 tunnels are widely used for modifying its catalytic activity. In this study, we reported a novel strategy of enhancing catalytic activity of OMS-2 for carcinogenic benzene elimination by tuning Sr2+ concentration in the tunnels. The catalytic activity result revealed that an obvious decrease (△T50 = 27 °C and △T90 = 37 °C) in T50 and T90 (corresponding to benzene conversions at 50 % and 90 %, respectively; initial benzene concentration was 2000 mg m-3; contact time was 1.5 s) had been observed by increasing the Sr2+ concentration in the OMS-2 tunnels. The origin of Sr2+ doping effect on catalytic activity was theoretically and experimentally investigated by CO temperature-programmed reduction, 18O2 isotope labeling, and density functional theory calculations. The result confirmed that increasing Sr2+ concentration in the tunnels not only promoted the lattice oxygen activity, but also facilitated the generation of more oxygen vacancy defects, thus considerably improving the catalytic activity of OMS-2.

The remarkable effect of alkali earth metal ion on the catalytic activity of OMS-2 for benzene oxidation.

Cryptomelane-type octahedral molecular sieve (OMS-2) is one of the most promising catalysts for VOCs oxidation, and introduction of metal ions in OMS-2 tunnel is widely used for tailoring its catalytic activity. Here, different types of alkali earth metal ions with the same X/Mn atomic ratio of approximately 0.012 (X represents Mg2+, Ca2+, and Sr2+) were successfully introduced into OMS-2 tunnel by a one-step redox reaction. The catalytic test showed that introducing alkali earth metal ions into tunnels had a considerable effect on the catalytic performance of OMS-2 for benzene oxidation. The Sr2+ doped OMS-2 catalyst exhibited the better catalytic activity compared with those of Mg-OMS-2 and Ca-OMS-2 samples, and was also superior to a commercial 0.5% Pt/Al2O3 catalyst, as evidenced by its low reaction temperatures of T50 = 200 °C and T90 = 223 °C (corresponding to benzene conversions at 50% and 90%, respectively). The origin of the considerable effect of alkali earth metal doping on the catalytic activity of OMS-2 catalysts was experimentally and theoretically investigated by an 18O2 isotopic labeling experiment, CO temperature-programmed reduction, O2 temperature-programmed oxidation, and density functional theory calculations. The greatest catalytic activity of Sr-OMS-2 compared with those of Mg-OMS-2 and Ca-OMS-2 samples was attributed to its highest lattice oxygen activity as well as its largest surface area. By introducing alkali earth metal ions into the OMS-2 tunnel, we developed a low-cost and highly efficient catalyst that could be used as alternative to noble metal catalysts.