Metal behavior and soil quality changes induced by the application of tailor-made combined biochar: An investigation at pore water scale.

The remediation performance of biochar varies based on the biomass used for its production. Further innovation involves developing tailor-made biochar by combining different raw materials to compensate for the limitations of pure biochar. Therefore, tailor-made combined biochar produced from the co-pyrolysis of pig manure and invasive Japanese knotweed (P1J1), as well as biochars produced from these feedstocks separately, i.e., pure pig manure (PM) and pure Japanese knotweed (JK), were applied to Pb and As contaminated soil to evaluate the biochar-induced changes on soil properties, microbial activity, DOM, and metal and metalloids solubility at the soil pore water scale. Biochar application reduced soluble Pb, whereas enhanced the As mobility; the increased soil pH after biochar addition played a fundamental role in reducing the Pb solubility, as revealed by their significant negative correlation (r = -0.990, p < 0.01). In contrast, the release of dissolved P strongly influenced As mobilization (r = 0.949, p < 0.01), especially in P-rich PM and P1J1 treatments, while JK showed a marginal effect in mobilizing As. Soils treated with PM, P1J1, and JK mainly increased Gram-negative bacteria by 56 %, 52 %, and 50 %, respectively, compared to the control. Fluorescence excitation-emission matrix spectroscopy combined with parallel factor analysis identified three components in pore water DOM, C1 (long wavelength humic-like), C2 (short wavelength humic-like), and C3 (protein-like), which were dominant respectively in the P1J1, JK, and PM-added soil. A principal component analysis (PCA) confirmed that the PM and P1J1 had similar performance and were more associated with releasing P and Mg and specific DOM components (C1 and C3). Meanwhile, P1J1 supplemented soil OM/OC and K, similar to JK. The results of this study suggest that combined biochar P1J1 can comprehensively enhance soil quality, embodying the advantages of pure PM and JK biochar while overcoming their shortcomings.

Comparative uptake, translocation, and plant mediated transport of Tc-99, Cs-133, Np-237, and U-238 in Savannah River Site soil columns for the grass species Andropogon virginicus.

This study examines the ability of the grass species Andropogon virginicus to alter the subsurface transport and redistribution of a suite of radionuclides (99Tc, 133Cs (stable analog for 135Cs and 137Cs), 237Np, 238U) with varying chemical behaviors in a Savannah River Site soil via the use of vegetated and unvegetated soil columns. After an acclimation period, a small volume of solution containing all radionuclides was introduced into the columns via Rhizon© pore water sampling tubes. Plants were grown for an additional 4 weeks before shoots were harvested, and columns were prepared for sampling. Plant presence led to decreased radionuclide release from the columns, mainly due to radionuclide specific combinations of system hydrology differences resulting from plant transpiration as well as plant uptake. For the most mobile radionuclides, 99Tc followed by 237Np, plant presence resulted in significantly different soil concentration profiles between vegetated and unvegetated columns, including notable upward migration for 237Np in columns with plants. Additionally, plant uptake of 99Tc was the greatest of all the radionuclides, with plant tissues containing an average of 44 % of the 99Tc, while plant uptake only accounted for <2 % of 237Np and <0.5 % of 133Cs and 238U in the system. Although overall plant uptake of 133Cs and 238U were similar, the majority of 133Cs taken up by plants was associated with 133Cs already available in the aqueous phase while 238U uptake was mainly associated with the solid phase, meaning that plant activity resulted in a fraction of the native 238U being mobilized and thus, made available for plant uptake. Overall, this study quantified the influence of several plant-mediated physical and biogeochemical factors that have significant influence on radionuclide mobility and transport in this complex system which can be further utilized in future system or site-specific environmental transport and risk assessment models.

Dissolution and Vertical Transport of Uranium from Stable Mineral Forms by Plants as Influenced by the Co-occurrence of Uranium with Phosphorus.

Plants could mobilize (dissolution followed by vertical transport) uranium (U) from mineral forms that are otherwise stable. However, the variability of this plant-mediated mobilization of U as a function of the presence of various essential plant nutrients contained in these minerals remains unknown. A series of column experiments were conducted using Andropogon virginicus to quantify the vertical transport of U from stable mineral forms as influenced by the chemical and physical coexistence of U with the essential nutrient, phosphorus (P). The presence of plants significantly increased the vertical migration of U only when U was precipitated with P (UO2HPO4·4H2O; chernikovite) but not from UO2 (uraninite) that lacks any essential plant nutrient. The U dissolution was further increased when chernikovite co-occurred with a sparingly available form of P (FePO4) under P-limited growing conditions. Similarly, A. virginicus accumulated the highest amount of U from chernikovite (0.05 mg/g) in the presence of FePO4 compared to that of uraninite (no-P) and chernikovite supplemented with KH2PO4. These results signify an increased plant-mediated dissolution, uptake, and leaching of radioactive contaminants in soils that are nutrient deficient, a key factor that should be considered in management at legacy contamination sites.

The Uptake and Translocation of 99Tc, 133Cs, 237Np, and 238U Into Andropogon Virginicus With Consideration of Plant Life Stage.

Hydroponic uptake studies were conducted to evaluate the uptake and translocation of Tc, Cs (stable analog for Cs), Np, and U into established and seedling Andropogon virginicus specimens under controlled laboratory conditions. Plant specimens were grown in analyte-spiked Hoagland nutrient solution for 24 h, 3 d, and 5 d. Translocation to shoots was greatest for Tc and Cs, likely due to their analogous nature to plant nutrients, while U (and Np to a lesser extent) predominantly partitioned to root tissue with less extensive translocation to the shoots. Plant age contributed significantly to differences in concentration ratios for all nuclides in shoot tissues (p ≤ 0.024), with higher concentration ratios for seedling specimens. Additionally, duration of exposure was associated with significant differences in concentration ratios of Cs and Tc for seedlings (p = 0.007 and p = 0.030, respectively) while plant part (root or shoot) was associated with significant differences in concentration ratios of established plants (p < 0.001 for both nuclides). Statistically significant increases in radionuclide uptake in seedling specimens relative to established plants under controlled conditions suggests that, in addition to geochemical factors, plant life stage of wild grasses may also be an important factor influencing radionuclide transport in the natural environment.

Phosphorus Stress-Induced Changes in Plant Root Exudation Could Potentially Facilitate Uranium Mobilization from Stable Mineral Forms.

Apparent deficiency of soil mineral nutrients often triggers specific physio-morphological changes in plants, and some of these changes could also inadvertently increase the ability of plants to mobilize radionuclides from stable mineral forms. This work, through a series of sand-culture, hydroponics, and batch-equilibration experiments, investigated the differential ability of root exudates of Andropogon virginicus grown under conditions with variable phosphorus (P) availability (KH2PO4, FePO4, Ca3(PO4)2, and no P) to solubilize uranium (U) from the uranyl phosphate mineral Chernikovite. The mineral form of P, and hence the bioavailability of P, affected the overall composition of the root exudates. The lower bioavailable forms of P (FePO4 and Ca3(PO4)2), but not the complete absence of P, resulted in a higher abundance of root metabolites with chelating capacity at 72 hrs after treatment application. In treatments with lower P-bioavailability, the physiological amino acid concentration inside of the roots increased, whereas the concentration of organic acids in the roots decreased due to the active exudation. In batch dissolution experiments, the organic acids, but not amino acids, increase the dissolution U from Chernikovite. The root exudate matrix of plants exposed to low available forms of P induced a >60% increase in U dissolution from Chernikovite due to 5-16 times greater abundance of organic acids in these treatments. However, this was ca. 70% of the theoretical dissolution achievable by this exudate matrix. These results highlight the potential of using active management of soil P as an effective tool to alter the plant-mediated mobilization of U in contaminated soil.