Land application of biochar is increasingly being considered for potential agronomic and environmental benefits, e.g., enhancing carbon sequestration, nutrient retention, water holding capacity, and crop productivity; and reducing greenhouse gas emissions and bioavailability of environmental contaminants. However, little is known about the transport of biochar particles in the aqueous environment, which represents a critical knowledge gap because biochar particles can facilitate the transport of adsorbed contaminants. In this study, column experiments were conducted to investigate biochar particle transport and retention in water-saturated quartz sand. Specific factors considered included biochar feedstocks (wheat straw and pine needle), pyrolysis temperature (350 and 550 °C), and particle size (micrometer-particle (MP) and nanoparticle (NP)). Greater mobility was observed for the biochars of lower pyrolysis temperatures and smaller particle sizes. Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) calculations that considered measured zeta potentials and Lewis acid-base interactions were used to better understand the influence of pyrolysis temperature on biochars particle transport. Most biochars exhibited attractive acid-base interactions that impeded their transport, whereas the biochar with the greatest mobility had repulsive acid-base interaction. Nonetheless, greater retention of the MPs than that of the NPs was in contrast with the XDLVO predictions. Straining and biochar surface charge heterogeneity were found to enhance the retention of biochar MPs, but played an insignificant role in the biochar NP retention. Experimental breakthrough curves and retention profiles were well-described using a two-site kinetic retention model that accounted for depth-dependent retention at one site. Modeled first-order retention coefficients on both sites 1 and 2 increased with increasing pyrolysis temperature and particle size.

Carbonaceous materials are commonly used in agronomic and environmental applications primarily as geosorbents, but their redox properties that may affect biogeochemical reactions are rarely documented. Herein, the role of activated carbon (AC) mediating microbial reduction of ferrihydrite is studied. Our batch experiment results show that AC facilitated the reduction of ferrihydrite by Shewanella oneidensis MR-1, but the pretreatment of AC with HNO further increased the rate of reduction. The redox-active oxygen-containing functional groups in AC were found to be responsible for the enhancement of the microbial reduction of ferrihydrite. This conclusion was supported by the electrochemical evidence that showed that the electron exchange capacity (EEC) of AC was facilitated due to the presence of quinone/hydroquinone groups and strongly positively correlated with the content of C═O groups. Moreover, the coprecipitation of vivianite and siderite was found in the products in the presence of AC, but siderite only was present in the absence of AC. The proper identification of potential functional groups in AC-mediating electron transfer during microbial reduction of ferrihydrite provides insights into the mechanism of reaction and potential roles carbonaceous materials may play in biogeochemical redox processes and, consequently, the fate of contaminants in the environment.

Electrical properties of plasma membranes (PMs), partially controlled by the ionic composition of the exposure medium, play significant roles in the distribution of ions at the exterior surface of PMs and in the transport of ions across PMs. The effects of coexisting cations (commonly Al 3+ , Ca 2+ , Mg 2+ , H + , and Na + ) on the uptake and toxicity of these and other ions (such as Cu 2+ , Zn 2+ , Ni 2+ , Cd 2+ , and H 2 AsO 4 -) to plants were studied in terms of the electrical properties of PMs. Increased concentrations of cations or decreased pH in rooting media, whether in solution culture or in soils, reduced the negativity of the electrical potential at the PM exterior surface (c 0 o ). This reduction decreased the activities of metal cations at the PM surface and increased the activities of anions such as H 2 AsO 4 -. Furthermore, the reduced c 0 o negativity increased the surface-to-surface transmembrane potential difference, thus increasing the electrical driving force for cation uptake and decreasing the driving force for anion uptake across PMs. Analysis of measured uptake and toxicity of ions using electrostatic models provides evidence that uptake and toxicity are functions of the dual effects of c 0 o (i.e. altered PM surface ion activity and surface-to-surface transmembrane potential difference gradient). This study provides novel insights into the mechanisms of plant-ion interactions and extends current theory to evaluate ion bioavailability and toxicity, indicating its potential utility in risk assessment of metal(loid)s in natural waters and soils.Some solutes in growth media, such as cations and organic matter, influence the bioavailability and toxicity of metals in natural waters and soils (Peijnenburg et al., 1997;Weng et al., 2004;Kopittke et al., 2010). Novel insights into the bioavailability and toxicity of metals have inspired the development of models in order to allow accurate impact assessments of metals emitted into the environment. The biotic ligand model (BLM; Di Toro et al., 2001), as an extension of the free ion activity model (FIAM), incorporates site-specific competitions among cations (commonly Ca 2+ , Mg 2+ , and H + ) and ionic toxicants (commonly heavy metals) for binding to a biotic ligand at the cell surface. The scientific and regulatory communities have become interested in the BLM and have incorporated it into regulations. However, the BLM as the main determinant of toxicant bioavailability does not deserve uncritical acceptance, and the mechanism of the ameliorative effectiveness of cations must be considered carefully, especially in light of cation enhancement of anion toxicity (Kinraide, 2006). Previous studies (Kinraide, 2006;Wang et al., 2008) showed that global electrostatic interactions at the plasma membrane (PM) exterior surfaces, rather than site-specific mechanisms, may play the dominant role in the phytotoxicity of metals.The process of metal uptake typically encompasses diffusion of the ion to the cell surface, speciation reactions, electrostatic interac...

Perfluorooctanesulfonate (PFOS) is a toxic and persistent organic pollutant that can be widely detected in agricultural soils. In this study, two lettuce cultivars with low PFOS accumulation were screened out to reduce the exposure of PFOS to the human body via vegetable consumption. The screened low-PFOS cultivars may help to ensure food safety, despite planting in highly PFOS-polluted soils (1.0 mg/kg), due to their high tolerance to PFOS and 4.4–5.7 times lower shoot PFOS concentration than the high-PFOS cultivars. Protein content and protein-mediated transpiration played key roles in regulating PFOS accumulation in the lettuce cultivars tested. Lower protein content, lower stomatal conductance, and lower transpiration rate resulted in low PFOS accumulation. This study reveals the mechanism of forming low-PFOS accumulation of lettuce cultivars at physiological and biochemical levels and lays a foundation for developing a cost-effective and safe approach to grow vegetables in PFOS-polluted soils.

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Biochar is the carbon-rich product obtained from the thermochemical conversion of biomass under oxygen-limited conditions. Biochar has attained extensive attention due to its agronomical and environmental benefits in agro-ecosystems. This work adopts the scientometric analysis method to assess the development trends of biochar research based on the literature data retrieved from the Web of Science over the period of 1998-2018. By analysing the basic characteristics of 6934 publications, we found that the number of publications grew rapidly since 2010. Based on a keyword analysis, it is concluded that scholars have had a fundamental recognition of biochar and preliminarily found that biochar application had agronomic and environmental benefits during the period of 1998-2010. The clustering results of keywords in documents published during 2011-2015 showed that the main research hotspots were "biochar production", "biochar and global climate change", "soil quality and plant growth", "organic pollutants removal", and "heavy metals immobilization". While in 2016-2018, beside these five main research hotspots, "biochar and composting" topic had also received greater attention, indicating that biochar utilization in organic solid waste composting is the current research hotspot. Moreover, updated reactors (e.g., microwave reactor, fixed-bed reactor, screw-feeding reactor, bubbling fluidized bed reactor, etc.) or technologies (e.g., solar pyrolysis, Thermo-Catalytic Reforming process, liquefaction technology, etc.) applied for efficient energy production and modified biochar for environmental remediation have been extensively studied recently. The findings may help the new researchers to seize the research frontier in the biochar field.

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