Effects of Modification and Co-Aging with Soils on Cd(II) Adsorption Behaviors and Quantitative Mechanisms by Biochar

In this study, original rice straw biochar and two KMnO4-modified biochars (pre- and postmodification) were prepared, which were all pyrolysed at 400℃. Premodified biochar had the largest Cd adsorption capacity, strongest acid and solute buffering capacity, which benefited from the increase of carbonate content, specific surface area and the emergence of Mn(II) and MnOx through modification. Original and premodified biochars were then conducted four types of aging process, namely, aging without soil, co-aging with acid (pH=5.00), neutral (pH=7.00) and alkaline (pH=8.30) soils, using an improved three-layer mesh method. The adsorption capacities of modified biochar were always larger than those of original biochar after aging processes. After four aging processes, Cd(II) adsorption capacities were basically in the order of aged biochar without soil > biochar co-aged with alkaline soil > biochar co-aged with neutral soil > biochar co-aged with acid soil, and KMnO4-modified biochar was always better than original biochar after co-aging with soils. The dominant adsorption mechanism of original and premodified biochars (fresh and aged) for Cd(II) was all the precipitation and adsorption with minerals (accounted for 58.55%~85.55%). In this study, we highlighted that biochar remediation for Cd should be evaluated by co-aging with soil instead of aging without soil participation.

Transition metal on topological chiral semimetal PdGa with tailored hydrogen adsorption and reduction

The difficulties in designing high-performance hydrogen evolution reaction (HER) catalysts lie in the manipulation of adsorption behaviors of transition metals (TMs). Topological chiral semimetals with super-long Fermi arc surface states provide an ideal platform for engineering the catalytic performance of TMs through the metal-support interaction. We found the adsorption trends of TMs can be modified significantly when deposited at the surface of the PdGa chiral crystal. The electron transfer from the TMs to the surface states of the PdGa reshapes the d band structure of TMs and weakens the hydrogen intermediate bonding. Especially, W/PdGa is expected to be a good HER catalyst with close to zero Gibbs free energy. Experimentally, we found a Pt-like exchange current density and turnover frequency when depositing W atoms at the PdGa nanostructures surface. The findings provide a way to develop high-efficient electrocatalysts by the interplay between topological surface states and metal-support interaction.

Extracellular Polymeric Substances Facilitate the Adsorption and Migration of Cu2+ and Cd2+ in Saturated Porous Media

Heavy metal contamination in groundwater is a serious environmental problem. Many microorganisms that survive in subsurface porous media also produce extracellular polymeric substances (EPS), but little is known about the effect of these EPS on the fate and transport of heavy metals in aquifers. In this study, EPS extracted from soil with a steam method were used to study the adsorption behaviors of Cu2+ and Cd2+, employing quartz sand as a subsurface porous medium. The results showed that EPS had a good adsorption capacity for Cu2+ (13.5 mg/g) and Cd2+ (14.1 mg/g) that can be viewed using the Temkin and Freundlich models, respectively. At a pH value of 6.5 ± 0.1 and a temperature of 20 °C, EPS showed a greater affinity for Cu2+ than for Cd2+. The binding force between EPS and quartz sand was weak. The prior saturation of the sand media with EPS solution can significantly promote the migration of the Cu2+ and Cd2+ in sand columns by 8.8% and 32.1%, respectively. When treating both metals simultaneously, the migration of Cd2+ was found to be greater than that of Cu2+. This also demonstrated that EPS can promote the co-migration of Cu2+ and Cd2+ in saturated porous media.