Thermal stability of biochar and its effects on cadmium sorption capacity

In this study, the thermal stability of a wood shaving biochar (WS, 650 °C), a chicken litter biochar (CL, 550 °C) and an activated carbon (AC, 1100 °C) were evaluated by combustion at 375 °C for 24 h to remove the labile non-carbonized organic matter. Results showed that WS and CL biochars were not thermally stable and can lose most of the organic C during combustion. The combusted WS and CL biochars retained considerable amounts of negative charge and displayed higher sorption for Cd (from 5.46 to 68.9 mg/g for WS and from 48.5 to 60.9 mg/g for CL). The AC retained 76.5% of its original C and became more negatively chargely after combustion, but its sorption for Cd slightly decreased (from 18.5 to 14.9 mg/g). This study indicated that after potential burning in wildfires (200 - 500 °C), biochars could have higher sorption capacity for metals by remaining minerals

Pyrogenic carbon in Australian soils

Pyrogenic carbon (PyC), the combustion residues of fossil fuel and biomass, is a versatile soil fraction active in biogeochemical processes. In this study, the chemo-thermal oxidation method (CTO-375) was applied to investigate the content and distribution of PyC in 30 Australian agricultural, pastoral, bushland and parkland soil with various soil types. Soils were sampled incrementally to 50 cm in 6 locations and at another 7 locations at 0–10 cm. Results showed that PyC in Australian soils typically ranged from 0.27–5.62 mg/g, with three Dermosol soils ranging within 2.58–5.62 mg/g. Soil PyC contributed 2.0–11% (N = 29) to the total organic carbon (TOC), with one Ferrosol as high as 26%. PyC was concentrated either in the top (0–10 cm) or bottom (30–50 cm) soil layers, with the highest PyC:TOC ratio in the bottom (30–50 cm) soil horizon in all soils. Principal component analysis - multiple linear regression (PCA-MLR) suggested the silt-associated organic C factor accounted for 38.5% of the variation in PyC. Our findings suggest that PyC is an important fraction of the TOC (2.0–11%, N = 18) and chemically recalcitrant organic C (ROC) obtained by chemical C fractionation method accounts for a significant proportion of soil TOC (47.3–84.9%, N = 18). This is the first study comparing these two methods, and it indicates both CTO-375 and C speciation methods can determine a fraction of recalcitrant organic C. However, estimated chemically recalcitrant organic carbon pool (ROC) was approximately an order of magnitude greater than that of thermally stable organic carbon (PyC)

Copper geochemistry in an acidic, sandy soil: sorption-desorption, aqueous speciation and mobility

The geochemical behavior of Cu in an acidic, sandy Podosol was examined. Sorption-desorption of Cu exhibited maximum linear distribution coefficients (Kd) at approx. pH 5. Observed depression of Kd values at pH \u3e 5, were attributed to increased solubility of native Dissolved Organic Carbon (DOC) at higher pH and subsequent formation of poorly-sorbing Cu-DOC complexes. Speciation modelling with the MINTEQA2 code indicated that \u3e 90 % of aqueous Cu was present as Cu-DOC complexes at pH \u3e 5.5. The effect of Cu loading was examined with sorption isotherm analysis at pH 5 using both constant (1:2 and 1:10) and variable solid:solution ratio approaches. As the solid:solution ratio increased, the proportion of Cu sorbed decreased due to the formation of Cu-DOC complexes. However, this effect was negligible once these Cu-DOC complexes were accounted for via free Cu2+ sorption isotherms. At a 1:10 solid:solution ratio, Cu sorption was described by a linear distribution coefficient at low sorption levels (Kd[low]) of 481 L/kg and a sorption capacity (CS,Max) of 382 mg/kg. Selective removal of soil organic matter reduced these values by approx. 95 %, indicating that Cu was sorbed predominantly to soil organic matter. The Kd[low] and CS,Max values from Cu desorption experiments were 934 L/kg and 516 mg/kg, respectively, which indicates that sorption was not fully reversible. This irreversibility was related to aqueous Cu speciation, showing that aqueous complexes between Cu and DOC comprised 28.3 to 72.8 % and 21.3 to 45.4 % of aqueous Cu in the sorption and desorption experiment, respectively. Sorption irreversibility was not evident when the corresponding data was presented as free Cu2+ isotherms. Both sorption and desorption experiments with free Cu2+ \u3c 0.2 mg/L were described by a Kd[low] value of approx. 3000 L/kg. The results show that many aspects of Cu behavior (including sorption-desorption hysteresis) can be explained by considering aqueous speciation

Phytocapping an alternative technology for the sustainable management of landfill sites /

Landfill remains the predominant means of waste disposal throughout the globe. Numerous landfills exist in developed and underdeveloped countries, engineered with contrasting degrees of effectiveness. Modern landfill closure in developed countries involves the conventional capping of waste with materials such as compacted clay or geosynthetic clay liners, typically overlain with other soil materials. Conventional capping technologies are now accepted to be increasingly ineffective in reducing percolation into waste. Cost-effective alternative systems are of increasing interest, including the use of plants to control and limit water entry into waste, otherwise known as “Phytocapping”. Phytocapping reduces percolation through three main mechanisms: (a) canopy interception of rainfall, (b) storage of moisture in the soil layers, and (c) evapotranspiration (i.e., hydraulic lift) of stored water. Phytocapping has been shown to be at least as effective as clay capping in reducing percolation through landfill cover materials, provided site specific conditions are factored into design, and providing many additional benefits, including increased cap stability, reduced erosion of capping materials, reduction of wind-blown dust, enhanced biological diversity, increased opportunity to establish commercial plants, carbon sequestration, and enhanced methane oxidation from microbial communities. Phytocapping has been suggested as having potential in phytoremediation of landfill leachate. The most common phytocapping approach to date is the construction of vegetation assemblages for the purposes of creating natural vegetation nodes. Phytocapping technology can be enhanced by appropriate selection of soil amendments such as biosolids, biochar, compost, or other materials. Appropriate selection of plant species and soil amendment products can enhance methane oxidation in capping soils. There is considerable potential for the use of high biomass energy plants but further work is needed in choosing appropriate plant species that will serve both purposes of site water balance aswell as commercial (e.g., timber, bioenergy) and biodiversity needs of the community

Bioavailability of barium to plants and invertebrates in soils contaminated by barite

Barium (Ba) is a nonessential element to terrestrial organisms and is known to be toxic at elevated concentrations. In this study, the bioavailability and toxicity of Ba in barite (BaSO4) contaminated soils was studied using standard test organisms (Lactuca sativa L. “Great Lakes”, Eisenia fetida). Contamination resulted from barite mining activities. Barium concentrations in contaminated soils determined by X-ray fluorescence were in the range 0.13–29.2%. Barite contaminated soils were shown to negatively impact both E. fetida and L. sativa relative to control soil. For E. fetida, pore-water concentrations and acid extractable Ba were linearly related to % body weight loss. In L. sativa, pore-water Ba and exchangeable Ba were both strongly related to shoot Ba and shoot biomass production. A negative linear relationship was observed between shoot Ba content and shoot weight (P < 0.0004, R2 = 0.39), indicating that Ba accumulation is likely to have induced phytotoxicity. Plant weights were correlated to % weight loss in earthworm (r = −0.568, P = 0.028). Barium concentrations in pore-water were lower than predicted from barite solubility estimates but strongly related to exchangeable Ba, indicating an influence of ion exchange on Ba solubility and toxicity to E. fetida and L. sativa.

Competitive sorption of cadmium and zinc in contrasting soils

The sorption behavior of cadmium (Cd(II)) and zinc (Zn(II)) on two virgin soils with different pH levels was studied using single metal and competitive dual metal systems. In the single metal system, Zn exhibited a greater affinity for the alkaline soil, as indicated by the Langmuir constant (KL = 8.85 L/kg) compared with Cd (KL = 1.79 L/kg). However, much less sorption of both Zn (KL = 0.19 L/kg) and Cd (KL = 0.07 L/kg) was observed in the acidic soil. The competitive sorption data were modeled using two-metal Freundlich and Langmuir functions. The competition for metal sorption occurred in the alkaline soil only at a higher concentration of the competing metals, whereas the effect was significant even at lower concentrations in the acidic soil. The cumulative amount of both metals sorbed in the soil was similar to that of single metal systems in the studied concentration range, demonstrating that the number of sites available for sorption remained constant irrespective of the competition. This study indicated that Cd might be more mobile in a mixed-metal system than in a single-metal scenario and thus poses a serious ecotoxicological threat. This study is important for assessing the risks and developing management strategies for multiple heavy metal contaminated soils

Pore-water carbonate and phosphate as predictors of arsenate toxicity in soil

Phytotoxicity of inorganic contaminants is influenced by the presence of competing ions at the site of uptake. In this study, interaction of soil pore-water constituents with arsenate toxicity was investigated in cucumber (Cucumis sativa L) using 10 contrasting soils. Arsenate phytotoxicity was shown to be related to soluble carbonate and phosphate. The data indicated that dissolved phosphate and carbonate had an antagonistic impact on arsenate toxicity to cucumber. To predict arsenate phytotoxicity in soils with a diverse range of soil solution properties, both carbonate and phosphate were required. The relationship between arsenic and pore-water toxicity parameters was established initially using multiple regression. In addition, based on the relationship with carbonate and phosphate we successively applied a terrestrial biotic ligand-like model (BLM) including carbonate and phosphate. Estimated effective concentrations from the BLM-like parametrization were strongly correlated to measured arsenate values in pore-water (R2 = 0.76, P \u3c 0.001). The data indicates that an ion interaction model similar to the BLM for arsenate is possible, potentially improving current risk assessments at arsenic and co-contaminated soils

Natural attenuation of Zn, Cu, Pb and Cd in three biosolids-amended soils of contrasting pH measured using rhizon pore water samplers

The effects of application of biosolids, at four rates, to an alkaline (pH 8.4), neutral (pH 7.0) and acidic (pH 4.0) soil on concentrations of Cu, Zn, Pb, Cd and dissolved organic C in soil solution were measured over a 170-day period in a laboratory incubation study using Rhizon pore water samplers. Applications of biosolids decreased solution pH in the alkaline soil, increased it in the acidic soil and had little effect in the neutral soil. In general, increasing application rates of biosolids progressively increased EC and concentrations of dissolved organic C (DOC), Cu, Zn, and to a lesser extent Cd and Pb, in soil solution. Concentrations of DOC and concentrations of solution Cu, Zn, and to a lesser extent solution Cd and Pb, decreased over the incubation period. In all three soils, concentrations of solution Cu and Zn were closely positively correlated with DOC concentrations and similar positive but weaker correlations were found for solution Cd and Pb. For the alkaline and neutral soils, concentrations of solution Cu, Zn, Cd and Pb were generally negatively correlated with solution pH but for the acidic soil, positive correlations for Cu and Zn were recorded. The percentage reduction in solution Cu and Zn, between 0 and 170 days incubation, increased with increasing rates of biosolids in the acid soil (where biosolids applications increased pH) but the reverse was the case for the alkaline soil (where pH fell following biosolids applications). Greatest percentage reduction in soluble Cu and Zn occurred in the neutral soil which had the greatest BET surface area, clay and organic matter contents and therefore the greatest capacity to adsorb heavy metal cations. It was concluded that solution pH, dissolved organic C and the intrinsic capacity of the soil to remove metals from solution, were the main factors interacting to regulate heavy metal cation solubility in the biosolids-amended soils