Land Application Of Biochar Research Articles

Consequential versus attributional life cycle optimization of poultry manure management technologies in a food-energy-water-waste nexus

To model the environmental and economic tradeoffs of managing poultry manure wastes in a food-energy-water-waste nexus, this work builds a framework to conduct a combined life cycle and techno-economic optimization and a spatial analysis of poultry manure management technologies. To align with the United Nations Sustainable Development Goals 6: Clean Water and Sanitation and 13: Climate Action by mitigating the massive water and climate impacts induced by the common practice of fertilizer crops using direct land application of poultry manure, the framework minimizes freshwater eutrophication (FE) impact and climate change impact (GWP). To account for the circular economy, the net present value (NPV) is maximized to measure the economic feasibility of implementing alternative management pathways. A multi-objective mixed-integer non-linear programming framework accounts for the selection of direct land application, and waste to energy technologies, spatial variation in manure supply, facility sizing, and the products’ consumption and end-of-life. This optimization framework is evaluated for a case study in New York state. Under an attributional life cycle optimization, anaerobic digestion minimizes FE with an impact of 0.002 kg P-eq/ton manure, while using a consequential life cycle optimization, hydrothermal carbonization minimizes FE with an impact of −0.96 kg P-eq/ton manure. For both life cycle methodologies, pyrolysis with land application of biochar minimizes GWP. At the attributional life cycle assessment tradeoff point for GWP and NPV, minimum carbon credits of US$33/ton CO2-eq are necessary for a pyrolysis plant to have a positive NPV, implying that harnessing economies of scale or robust carbon accounting is necessary for the widespread adoption of pyrolysis as a poultry manure management technology.

Biochar nanoparticles-mediated transport of organic contaminants in porous media: dependency on contaminant properties and effects of biochar aging

Land application of biochar has been recommended as an effective soil amendment measure. Nonetheless, the applied biochar can accumulate co-existing contaminants. Meanwhile, nanoparticles formed due to biochar disintegration may facilitate contaminant transport in vadose zone and groundwater, posing a potential risk to the subsurface environment. Here, we show that the presence of pinewood- and rice straw-derived biochar nanoparticles (BCNPs) at parts per million level (~ 20 mg/L) can result in significant mobilization of hydrophobic, nonpolar contaminants (naphthalene and pyrene) and positively charged polar contaminants (trimethoprim and ciprofloxacin) in saturated sandy soil, but slightly inhibits the transport of negatively charged or neutral hydrophilic compounds (sulfamethoxazole and bisphenol A). With supplemental adsorption and desorption experiments we show that the ability of BCNPs in mediating contaminant transport (either enhancing or inhibiting) relies primarily on the extent of irreversible binding of a contaminant to the BCNPs. Sulfide reduction and leaching of organic carbon, two relatively mild (in terms of modification of physicochemical properties, e.g., surface O/C ratio) but widely occurring aging processes, facilitate co-transport of pyrene and bisphenol A with the BCNPs. However, this is mainly the result of increased mobility of the BCNPs (i.e., the carrier), rather than enhanced interactions between the BCNPs and the contaminants being carried. The findings underline the significant effects of BCNPs on the fate and transport of environmental contaminants, and further highlight the important role of aging in affecting environmental behaviors and effects of biochar materials.Graphical

Co-pyrolysis of biosolids with alum sludge: Effect of temperature and mixing ratio on product properties

Sustainable management of biosolids and alum sludge are becoming increasingly important due to the rapid growth in their production with increasing urbanization globally. Pyrolysis has been identified as a promising method for biosolids management. However, land application of biochar derived from pyrolysis of biosolids may be limited despite their significant agro-environmental benefits, mainly due to the potential increased concentration of heavy metal content in the biochar. Current study experimentally investigated co-pyrolysis of biosolids with alum sludge with an aim to produce high quality biochar with reduced heavy metal content. Moreover, it is further hypothesized that the addition of alum sludge may catalyze pyrolysis reactions, which may alter properties of pyrolysis products such as oil, gas and biochar. The results indicate that co-pyrolysis can reduce heavy metal concentrations in biochar while also improving oil and gas product qualities, suggesting beneficial synergistic effects. Furthermore, the process parameters such as pyrolysis temperature and alum sludge mixing ratio have a significant impact on the yield distribution and product (biochar, pyrolysis oil and gas) properties.

Rice Rhizospheric Effects on the Bioavailability of Toxic Trace Elements during Land Application of Biochar.

Land application of biochar, the product of organic waste carbonization, can improve soil fertility as well as sequester carbon to mitigate climate change. In addition, biochar can greatly influence the bioavailability of toxic trace elements (TTEs) in soils resulting from its large internal surface areas, abundance in organic carbon, and ability to modify soil pH. Most research to date employs batch leaching tests to predict how biochar addition impacts TTE bioavailability, but these ex situ tests rarely considered the rhizospheric effect which might offset or intensify the changes induced by organic residue addition. This is especially so in rice rhizospheres because of strong clines in localized redox conditions. In this study, we adopted in situ high-resolution (HR) diffusive gradients in thin films (DGT) as well as rhizo-bag porewater sampling experiments to depict an overall picture of the difference in TTE (As, Cd, Cu, Ni, and Pb) bioavailability between the rice rhizosphere and bulk soils during land application of biochar. Porewater sampling experiments revealed that biochar additions stimulated TTE release due to the increase of dissolved organic carbon (DOC) and H+ concentrations. In the rhizosphere, although biochar still promoted As, Cd, and Ni release into porewaters, the rhizospheric effect was one of dampening/reduction compared with the bulk soil. When we focused on the localized changes of TTE bioavailability in the rhizosphere using an in situ HR-DGT approach, on the contrary, flux maxima of Cd, Cu, and Ni occurred near/on the root surface, and hot spots of As can be observed at peripheries of the rooting zone, which demonstrated the high heterogeneity and complexity of the rhizosphere's influence on TTE bioavailability.

Land application of sewage sludge biochar: Assessments of soil-plant-human health risks from potentially toxic metals

Effects of sewage sludge-derived biochar and its precursor on the accumulation of metals (Cd, Cu, Pb and Zn) in soil and their uptake by plants in a 1-year field experiment involving corn-radish rotation were comparatively studied. The human health risks were assessed, and the safe application period of biochar were estimated. The application of biochar, compares to sewage sludge, significantly enhanced the radish yield (p < 0.05; not corn yield) and significantly reduced the accumulation of metals in both plants (p < 0.05), especially the annual application at ≤15 t ha−1. The hazard quotient analyses of the metals showed there were no health risks to humans (Hazard Index < 1) in consuming the edible parts of the both plants. The application of sewage sludge at ≥15 t ha−1 resulted in Cd in radish exceeded the threshold for foodstuffs set by China (0.1 mg kg−1). The total contents of Cd, Cu, Pb and Zn in soil increased gradually as the application of sewage sludge or its biochar increased from 7.5 t ha−1 to 30 t ha−1. More metals were found to be introduced to soil by the land application of biochar than by its precursor at the same doses, because the metals were concentrated in biochar during the preparation process. The contamination risk assessment of soil based on the geo-accumulation index, the contamination factor and the pollution load index suggested the application of biochar on farmland should <15 t ha−1. Therefore, taking into account the yield of and metal concentrations in the radish and corn plants and the contamination risks in soil, it is recommended that the continuous safe application period at an application of 7.5 t ha−1 year−1 of biochar should not exceed 15 years, and that of its precursor sewage sludge should not exceed 17 years.

Evaluation of the Bioavailability of Heavy Metals and Phosphorus in Biochar Derived from Manure and Manure Digestate

The effects of pyrolysis temperature (350, 450, and 550 °C) on the properties of biochar prepared from chicken manure (CM), dairy manure (DM), and their digestates (CMD and DMD) were investigated in this study. The physicochemical properties and the transformation of phosphorous and heavy metal forms in various types of biochar were analyzed, and the bioavailability was assessed to optimize the pyrolysis condition towards biochar land application. The larger specific surface area was found in CM and CMD derived biochars (14.90–22.45 m2/g), as compared to DM and DMD derived biochars (1.17–7.36 m2/g). The highest contents of total phosphorous (TP) and bioavailable non-apatite inorganic phosphorus (NAIP) were obtained in DMD biochar, i.e., 49.31 and 27.03 mg/g TS, respectively. Cu and Zn are identified as the harmful heavy metal elements in manure derived biochars due to its high level of total concentration. When increasing pyrolysis temperature, the fractions of Zn and Cu in exchangeable-, carbonate-, and the organic-bonded state decreased and the fractions of manganese oxidized and residual state increased. Finally, the citric acid leaching treatment was proposed to decrease the Zn and Cu contents in biochar before land application. The overall leaching rates for Cu and Zn were 37–45% and 27–32%, respectively.

Physicochemical property and colloidal stability of micron- and nano-particle biochar derived from a variety of feedstock sources

The ever-increasing land application of biochar may raise the environmental issue of micronparticle (MP) and nanoparticle (NP) biochars for their high mobility or as a carrier to facilitate transport of contaminants in soil. In this study, a variety of biochars were produced from pyrolysis of nine biomass sources and then subjected to the extraction of MP and NP biochars. The diverse physicochemical properties and electrokinetic stability of MP and NP biochars were further investigated. MP and NP biochars accounted for 1.43–20.5% and 0.99–15.3% of bulk biochar and had colloidal particle diameters mainly smaller than 1 μm and 100 nm, respectively. The MP and NP biochars contained more O-containing functional groups and mineral components but less aromatic clusters than bulk biochar. The yield of MP/NP biochars derived from plant sources such as woods, herbs, and agricultural waste was positively linear to the ash content of their bulk biochars but this relationship wasn't applied to the municipal sourced biochar such as manure and sewage sludge. More condensed aromatic rings and functional groups were found in MP/NP biochar from plant biomass than municipal sourced biochar. However, the latter was rich with minerals like carbonates, phosphates, and silicates. Higher functional groups in the plant sourced MP/NP wheat straw biochar accounted for the extremely high stability to resist the whole range of ionic strength studied, while the municipal sourced MP/NP dairy manure biochar with less functional groups and more minerals were readily destabilized, with the Critical Coagulation Concentration (CCC) values of 75 mM and 100 mM, respectively. Overall, this study revealed the size-dependent characteristics of composition and structure as well as high colloidal stability of MPs and NPs which are helpful for prediction of their environmental fate and risk.

Contrasting effects of biochar nanoparticles on the retention and transport of phosphorus in acidic and alkaline soils

Land application of biomass-derived biochar has been increasingly recommended as a beneficial soil amendment for nutrients (such as N, P) retention. However, the small-scale biochar particles, especially those in the nano-scale range, may carry nutrients downward the soil profile, reducing nutrition retention and posing a potential risk to the groundwater. In this study, column experiments were conducted to investigate the retention and transport of phosphorus (P) in two acidic and two alkaline soils as affected by wood chip-derived biochar nanoparticles (NPs). In acidic paddy and red soils, biochar NPs facilitated the retention of P, increasing by about 24% and 16%, respectively, compared to the biochar absence. It is because biochar NPs stabilize soil Fe/Al oxides and dissolved organic carbon (DOC), thereby reducing the release of Fe/Al oxides- and DOC-associated P. In contrast, in alkaline huangmian and chao soils, retention of P was reduced in the presence of biochar NPs, decreasing by about 23% and 18%, respectively. It was mainly due to the increased transport of Fe/Al oxides-associated P in effluents. Moreover, biochar NPs could also act as a P carrier, mediating the retention of P. The diffusive gradients in thin films provided in-suit measurement of labile P in soil profiles, showing much lower labile P from retained P in acidic soils than that from alkaline soils though the labile P with biochar NPs presence was increased in all soils. Our findings indicate that biochar NPs have contrasting effects on the retention of P in acidic and alkaline soils, implying the cautious land applications of biochar for nutrients retention in soils with different acidities.

Transport and retention of biochar nanoparticles in a paddy soil under environmentally-relevant solution chemistry conditions

Land application of biochar has been increasingly recommended as a powerful strategy for carbon sequestration and soil remediation. However, the biochar particles, especially those in the nanoscale range, may migrate or carry the inherent contaminants along the soil profile, posing a potential risk to the groundwater. This study investigated the transport and retention of wood chip-derived biochar nanoparticles (NPs) in water-saturated columns packed with a paddy soil. The environmentally-relevant soil solution chemistry including ionic strength (0.10–50 mM), electrolyte type (NaCl and CaCl2), and natural organic matter (0–10 mg L−1 humic acid) were tested to elucidate their effects on the biochar NPs transport. Higher mobility of biochar NPs was observed in the soil at lower ionic strengths, with CaCl2 electrolyte being more effective than NaCl in decreasing biochar NPs transport. The retained biochar NPs in NaCl was re-entrained (∼57.7%) upon lowering transient pore-water ionic strength, indicating that biochar NPs were reversibly retained in the secondary minimum. In contrast, negligible re-entrainment of biochar NPs occurred in CaCl2 due to the primary minimum and/or particle aggregation. Humic acid increased the mobility of biochar NPs, likely due to enhanced electrosteric repulsive interactions. The transport behaviors of biochar NPs can be well interpreted by a two-site kinetic retention model that assumes reversible retention for one site, and irreversible retention for the other site. Our findings indicated that the transport of wood chip biochar NPs is significant in the paddy soil, highlighting the importance of understanding the mobility of biochar NPs in natural soils for accurately assessing their environmental impacts.

Particulate matter emissions from biochar-amended soils as a potential tradeoff to the negative emission potential.

Novel carbon sequestration strategies such as large-scale land application of biochar may provide sustainable pathways to increase the terrestrial storage of carbon. Biochar has a long residence time in the soil and hence comprehensive studies are urgently needed to quantify the environmental impacts of large-scale biochar application. In particular, black carbon emissions from soils amended with biochar may counteract the negative emission potential due to the impacts on air quality, climate, and biogeochemical cycles. We investigated, using wind tunnel experiments, the particulate matter emission potential of a sand and two agriculturally important soils amended with different concentrations of biochar, in comparison to control soils. Our results indicate that biochar application considerably increases particulate emissions possibly by two mechanisms–the accelerated emission of fine biochar particles and the generation and emission of fine biochar particles resulting from abrasion of large biochar particles by sand grains. Our study highlights the importance of considering the background soil properties (e.g., texture) and geomorphological processes (e.g., aeolian transport) for biochar-based carbon sequestration programs.

Life cycle cost and economic assessment of biochar-based bioenergy production and biochar land application in Northwestern Ontario, Canada

Replacement of fossil fuel based energy with biochar-based bioenergy production can help reduce greenhouse gas emissions while mitigating the adverse impacts of climate change and global warming. However, the production of biochar-based bioenergy depends on a sustainable supply of biomass. Although, Northwestern Ontario has a rich and sustainable supply of woody biomass, a comprehensive life cycle cost and economic assessment of biochar-based bioenergy production technology has not been done so far in the region. In this paper, we conducted a thorough life cycle cost assessment (LCCA) of biochar-based bioenergy production and its land application under four different scenarios: 1) biochar production with low feedstock availability; 2) biochar production with high feedstock availability; 3) biochar production with low feedstock availability and its land application; and 4) biochar production with high feedstock availability and its land application- using SimaPro®, EIOLCA® software and spreadsheet modeling. Based on the LCCA results, we further conducted an economic assessment for the break-even and viability of this technology over the project period. It was found that the economic viability of biochar-based bioenergy production system within the life cycle analysis system boundary based on study assumptions is directly dependent on costs of pyrolysis, feedstock processing (drying, grinding and pelletization) and collection on site and the value of total carbon offset provided by the system. Sensitivity analysis of transportation distance and different values of C offset showed that the system is profitable in case of high biomass availability within 200 km and when the cost of carbon sequestration exceeds CAD $60 per tonne of equivalent carbon (CO2e). Biochar-based bioenergy system is economically viable when life cycle costs and environmental assumptions are accounted for. This study provides a medium scale slow-pyrolysis plant scenario and we recommend similar experiments with large-scale plants in order to implement the technology at industrial scale.

Transport and retention of bacteria and viruses in biochar-amended sand

The transport and retention of Escherichia coli and bacteriophages (PRD1, MS2 and ФX174), as surrogates for human pathogenic bacteria and viruses, respectively, were studied in the sand that was amended with several types of biochar produced from various feedstocks. Batch and column studies were conducted to distinguish between the role of attachment and straining in microbe retention during transport. Batch experiments conducted at various solution chemistries showed negligible attachment of viruses and bacteria to biochar before or after chemical activation. At any given solution ionic strength, the attachment of viruses to sand was significantly higher than that of biochar, whereas bacteria showed no attachment to either sand or biochar. Consistent with batch results, biochar addition (10% w/w) to sand reduced virus retention in the column experiments, suggesting a potential negative impact of biochar application to soil on virus removal. In contrast, the retention of bacteria was enhanced in biochar-amended sand columns. However, elimination of the fine fraction (<60μm) of biochar particles in biochar-amended sand columns significantly reduced bacteria retention. Results from batch and column experiments suggest that land application of biochar may only play a role in microbe retention via straining, by alteration of pore size distribution, and not via attachment. Consequently, the particle size distribution of biochar and sediments is a more important factor than type of biochar in determining whether land application of biochar enhances or diminishes microbial retention.

Life cycle environmental impact assessment of biochar-based bioenergy production and utilization in Northwestern Ontario, Canada

Biochar-based bioenergy production and subsequent land application of biochar can reduce greenhouse gas emissions by fixing atmospheric carbon into the soil for a long period of time. A thorough life cycle assessment of biochar-based bioenergy production and biochar land application in Northwestern Ontario is conducted using SimaPro® Ver. 8.1. The results of energy consumption and potential environmental impact of biochar-based bioenergy production system are compared with those of conventional coal-based system. Results show that biochar land application consumes 4847.61 MJ per tonne dry feedstock more energy than conventional system, but reduces the GHG emissions by 68.19 kg CO2e per tonne of dry feedstock in its life cycle. Biochar land application improves ecosystem quality by 18 %, reduces climate change by 15 %, and resource use by 13 % but may adversely impact on human health by increasing disability adjusted life years by 1.7 % if biomass availability is low to medium. Replacing fossil fuel with woody biomass has a positive impact on the environment, as one tonne of dry biomass feedstock when converted to biochar reduces up to 38 kg CO2e with biochar land application despite using more energy. These results will help understand a comprehensive picture of the new interventions in forestry businesses, which are promoting biochar-based bioenergy production.

Wheat straw and its biochar had contrasting effects on soil C and N cycling two growing seasons after addition to a Black Chernozemic soil planted to barley

Application of crop residues and its biochar produced through slow pyrolysis can potentially increase carbon (C) sequestration in agricultural production systems. The impact of crop residue and its biochar addition on greenhouse gas emission rates and the associated changes of soil gross N transformation rates in agricultural soils are poorly understood. We evaluated the effect of wheat straw and its biochar applied to a Black Chernozemic soil planted to barley, two growing seasons or 15 months (at the full-bloom stage of barley in the second growing season) after their field application, on CO2 and N2O emission rates, soil inorganic N and soil gross N transformation rates in a laboratory incubation experiment. Gross N transformation rates were studied using the 15N isotope pool dilution method. The field experiment included four treatments: control, addition of wheat straw (30 t ha−1), addition of biochar pyrolyzed from wheat straw (20 t ha−1), and addition of wheat straw plus its biochar (30 t ha−1 wheat straw + 20 t ha−1 biochar). Fifteen months after their application, wheat straw and its biochar addition increased soil total organic C concentrations (p = 0.039 and <0.001, respectively) but did not affect soil dissolved organic C, total N and NH4 +-N concentrations, and soil pH. Biochar addition increased soil NO3 −-N concentrations (p = 0.004). Soil CO2 and N2O emission rates were increased by 40 (p < 0.001) and 17 % (p = 0.03), respectively, after wheat straw addition, but were not affected by biochar application. Straw and its biochar addition did not affect gross and net N mineralization rates or net nitrification rates. However, biochar addition doubled gross nitrification rates relative to the control (p < 0.001). Our results suggest that land application of biochar, as opposed to the application of the raw wheat straw, could suppress CO2 and N2O emissions and enhance soil C sequestration. However, the implications of the increased soil gross nitrification rate and NO3 −-N in the biochar addition treatment for long-term NO3 −-N dynamics and N2O emissions need to be further studied.

Geological Sequestration of Biomass Char to Mitigate Climate Change

L anthropogenic climate change requires developing new routes for energy supply with net negative emissions, such as bioenergy with carbon capture and storage (BECCS). To achieve this goal, many works have dealt with the land application of biochar or the sequestration of CO2 from biomass combustion. In my opinion, the concept of wood geostorage has not yet received enough attention. Here, I even propose to extend this concept to the geological sequestration of biochar (GSB), displayed on figure 1. I intend to open a debate on this concept and to argue on its potentials. (1). ABOUT BIOSPHERE MANAGEMENT AND THE POTENTIAL OF CARBON SEQUESTRATION BY BIOMASS

Antagonistic Effects of Humic Acid and Iron Oxyhydroxide Grain-Coating on Biochar Nanoparticle Transport in Saturated Sand

Biochar land application may result in multiple agronomic and environmental benefits (e.g., carbon sequestration, improving soil quality, and immobilizing environmental contaminants). However, our understanding of biochar particle transport is largely unknown in natural environments with significant heterogeneity in solid (e.g., patches of iron oxyhydroxide coating) and solution chemistry (e.g., the presence of natural organic matter), which represents a critical knowledge gap in assessing the environmental impact of biochar land application. Transport and retention kinetics of nanoparticles (NPs) from wheat straw biochars produced at two pyrolysis temperatures (i.e., 350 and 550 °C) were investigated in water-saturated sand columns at environmentally relevant concentrations of dissolved humic acid (HA, 0, 1, 5, and 10 mg L(-1)) and fractional surface coverage of iron oxyhydroxide coatings on sand grains (ω, 0.16, 0.28, and 0.40). Transport of biochar NPs increased with increasing HA concentration, largely because of enhanced repulsive interaction energy between biochar NPs and sand grains. Conversely, transport of biochar NPs decreased significantly with increasing ω due to enhanced electrostatic attraction between negatively charged biochar NPs and positively charged iron oxyhydroxides. At a given ω of 0.28, biochar NPs were less retained with increasing HA concentration due to increased electrosteric repulsion between biochar NPs and sand grains. Experimental breakthrough curves and retention profiles were well described using a two-site kinetic retention model that accounted for Langmuirian blocking or random sequential adsorption at one site. Consistent with the blocking effect, the often observed flat retention profiles stemmed from decreased retention rate and/or maximum retention capacity at a higher HA concentration or smaller ω. The antagonistic effects of HA and iron oxyhydroxide grain-coating imparted on the mobility of biochar NPs suggest that biochar colloid transport potential will be dependent on competitive influences exerted by a number of environmental factors (e.g., natural organic matter and metal oxides).

The effects of walnut shell and wood feedstock biochar amendments on greenhouse gas emissions from a fertile soil

Land application of biochar, as a strategy to enhance soil fertility and reduce greenhouse gas (GHG) emissions is receiving widespread interest. Short-term soil incubations (29days) were used to investigate the effects of agriculturally relevant biochar applications from two contrasting feedstocks and temperatures on CO2 and N2O emissions from a fertile agricultural soil amended with different types of fertilizer (organic and synthetic). In addition, the effects of biochar on the denitrification process were examined using an acetylene based method to ascertain N2O and N2 emissions during denitrification. Complementary incubation experiments without soil (biochar and biochar with compost) examined the impact on natural or amended organic matter (compost) and biochar stability and surface chemistry were also investigated. Batch incubations (25°C) of biochar (softwood pyrolyzed at 410°C [WF410] and 510°C [WF510] and walnut shell pyrolyzed at 900°C [WA900]) amended soils were performed to determine emissions of CO2 and N2O due to complete (absence of acetylene [C2H2]) and incomplete denitrification (presence of C2H2). Similarly, GHG emissions from the complementary incubations were also measured. Concurrent biochar surface compositional changes were investigated with attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy. Biochar effects on CO2 emissions were not significantly different from controls. WA900 biochar (high pH) affects N cycling resulting in significantly higher emissions of N2O under conditions of complete denitrification and of N2 under conditions examining incomplete denitrification. WF410 (highest H/C ratio and lowest surface area) treatments with compost resulted in higher GHGs emissions which is attributed to a priming effect of the compost organic matter (COM). In addition, WF410 was most susceptible to degradation, evident from infrared spectroscopic analysis of the biochars. Although these results suggest that not all biochars provide substantial benefits as a soil amendment, the data do demonstrate potential for development of biochars with beneficial impacts on GHG emission mitigation and enhancement of soil C stocks.

Transport of Biochar Particles in Saturated Granular Media: Effects of Pyrolysis Temperature and Particle Size

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.

Energy Balance and Emissions Associated with Biochar Sequestration and Pyrolysis Bioenergy Production

The implications for greenhouse gas emissions of optimizing a slow pyrolysis-based bioenergy system for biochar and energy production rather than solely for energy production were assessed. Scenarios for feedstock production were examined using a life-cycle approach. We considered both purpose grown bioenergy crops (BEC) and the use of crop wastes (CW) as feedstocks. The BEC scenarios involved a change from growing winter wheat to purpose grown miscanthus, switchgrass, and corn as bioenergy crops. The CW scenarios consider both corn stover and winter wheat straw as feedstocks. Our findings show that the avoided emissions are between 2 and 5 times greater when biochar is applied to agricultural land (2--19 Mg CO2 ha(-1) y(-1)) than used solely for fossil energy offsets. 41--64% of these emission reductions are related to the retention of C in biochar, the rest to offsetting fossil fuel use for energy, fertilizer savings, and avoided soil emissions other than CO2. Despite a reduction in energy output of approximately 30% where the slow pyrolysis technology is optimized to produce biochar for land application, the energy produced per unit energy input at 2--7 MJ/MJ is greater than that of comparable technologies such as ethanol from corn. The C emissions per MWh of electricity production range from 91-360 kg CO2 MWh(-1), before accounting for C offset due to the use of biochar are considerably below the lifecycle emissions associated with fossil fuel use for electricity generation (600-900 kg CO2 MWh(-1)). Low-temperature slow pyrolysis offers an energetically efficient strategy for bioenergy production, and the land application of biochar reduces greenhouse emissions to a greater extent than when the biochar is used to offset fossil fuel emissions.