The key concept is the production of a partially oxygenated biochar material, which possesses enhanced cation-exchanging property. The technology concept is based on our recent experimental finding that the oxygen (O) to carbon (C) atom (O:C) ratio in biochar material correlates with its cation-exchange capacity [8, 9]. This makes sense, since the cation-exchanging groups are generally oxygen-containing chemical groups, such as hydroxy (-OH) and carboxyl (-COOH) groups. The cation-exchanging ability of a biochar is known to be predominantly dependent on the density of cation-exchanging groups present in the biochar. This biochar CEC-enhancing technology comprises a series of methods described below.

In one of the proposed methods, a biochar source is reacted with one or more oxygenating compounds in a controlled manner such that the biochar source homogeneously acquires oxygen-containing cation-exchanging groups in an incomplete combustion process. An “incomplete combustion process” means that a significant portion of the total carbon content in the biochar source remains and is not converted to oxide gases of combustion after the oxygenation process is completed. Oxide gases of combustion typically include, for example, CO2, H2O, and CO. No more than about 20% by weight of the carbon contained in the biochar source should be lost due to combustion, and preferably, only between 1 and 10% by weight of the carbon contained in the biochar source is converted to combustion gases. The best yield of biochar can be obtained from a combustionless process, in which substantially none (e. g., less than 0.5% by weight) of the carbon content of the biochar source is lost through conversion to oxide gases.

In contrast to the typically used, highly uncontrolled combustion processes, the method described here uses a highly controlled oxygenation process that results in the partial oxidation of biochar material, such that a biochar with a cation exchange property could be produced while advantageously emitting much lower amounts of oxide gases of combustion. Moreover, due in large part to the controlled nature of the oxygenation process, this method produces a substantially uniform and homo­geneously oxygenated biochar. In this context, “substantially uniform” means that there is an absence in the oxygenated biochar of regions of nonoxygenated biochar (as commonly found in biochar material formed under uncontrolled conditions, such as in open pits). A substantially uniform oxygenated biochar will possess different macroscopic regions of 100 pm2 to 1 cm2 in size that vary by no more than

0. 1-10% in at least one characteristic, such as CEC, oxygen to carbon ratio, and/or surface area. The substantial uniformity of the oxygenated biochar advantageously provides a user with a biochar material that gives a consistent result when distributed into soil, either packaged or in the ground. Furthermore, a substantial uniformity of the oxygenated biochar ensures that a tested characteristic of the biochar is indicative of the entire batch of biochar.

This method ensures production of a substantially uniform biochar by an effective level of mixing of the biochar during the oxygenation process. For example, biochar can be agitated, shaken, or stirred either manually or mechanically during the oxygen­ation process. In another example, the biochar can be reacted in an open or closed container such as a kiln fitted with a tumbling mechanism such that the biochar is tumbled and mixed during the oxygenation reaction.

The biochar source can be any biochar material that could benefit by the oxygen­ation process of the inventive method. In addition to raw biomass, byproducts of a pyrolysis or gasification process, or material acquired from an existent biochar deposit can be used. Generally, the biochar is plant-derived (i. e., derived from cel — lulosic biomass or vegetation). The plant-derived biomass materials include: cornsto- ver (e. g., the leaves, husks, stalks, or cobs of corn plants), grasses (e. g., switchgrass, miscanthus, wheat straw, rice straw, barley straw, alfalfa, bamboo, hemp), sugarcane, hull or shell material (e. g., peanut, rice, and walnut hulls), woodchips, saw dust, paper or wood pulp, food waste, agricultural waste, and forest waste. The biomass material can be in its native form, i. e., unmodified except for natural degradation processes or the biomass material can be modified by adulteration with a non-biomass carbon — based material (e. g., plastic — or rubber-based materials) or by physical modification (e. g., mashing, grinding, compacting, blending, heating, steaming, bleaching, nitro- genating, oxygenating, or sulfurating), before being converted to biochar.

The oxygenating compounds that are used include any general use compounds or materials that tend to be reactive by imparting oxygen atoms into organic materials. Most notable in this regard is oxygen gas, in the form of air. The oxygen gas may also be in an artificial gas mixture, such as an oxygen-nitrogen, oxygen-argon, oxygen-helium, oxygen-steam (H2O), or oxygen-carbon dioxide mixture. An artificial gas mixture can be advantageous in that the level of oxygen can be precisely controlled, thereby preventing combustion and optimizing the density and kind of oxygen-containing groups in the biochar. Some examples of other oxygenating compounds include the hypohalites (e. g., a hypochlorite salt, such as NaOCl), the halites (e. g., a chlorite or bromite salt, such as NaO2Cl or NaO2Br), the halates (e. g., a chlorate or bromate salt, such as NaO3Cl or NaO3Br), the perhalates (e. g., a perchlorate, perbromate, or periodate salt, such as NaO4Cl, NaO4Br, or NaO4I), the halogen — oxygen compounds (e. g., QO44 , the peroxides (e. g., H2O2 and urea peroxide), superoxides (e. g., NaO2 and KO2), ozone, pyrosulfates (e. g., NaS2O7), peroxodisul — fates (e. g., Na2S2O7, K2S2O7 , and (NH4)2S2O7), percarboxylic acids (e. g., peracetic acid), percarbonates, and permanganates (e. g., K2MnO4). Alternatively, oxygenation can be carried out with two or more chemicals that react with each other to form oxygen gas in situ (e. g., a permanganate salt or hypohalite combined with hydrogen peroxide).

In another version of this method shown in Fig. 1, the biochar source is treated with an oxygen plasma. Any of the oxygen plasma processes, including high and low temperature plasma processes, can be used to introduce oxygen into the biochar materials to increase their O:C ratios, and, concomitantly, their CEC. Thus, the technological concept described here includes oxygen plasma treatment as a treatment that improves CEC by enhanced “partial oxygenation” of the biochar materials.

Preferably, a low temperature oxygen plasma (e. g., 15-30°C), commonly used for surface modification and cleaning, is used. Generally, the plasma process entails subjecting the biochar at reduced pressure (i. e., in a vacuum chamber) to a source of ionized oxygen or oxygen radicals. The ionized source of oxygen is typically pro­duced by exposing oxygen at a reduced pressure of about 0.05-2 Torr to an ionizing source, such as an ionizing microwave, radiofrequency, or current source. Commonly, a radiofrequency source (e. g., of 13.56 MHz at an RF power of about 10-100 W) is used to ionize the oxygen. The particular oxygen plasma conditions depend on several factors, including the type of plasma generator, gas composition, power source capability and characteristics, operating pressure and temperature, the degree of oxygenation required, and characteristics of the particular biochar being treated (i. e., its susceptibility or resistance to oxygenation). Depending on several factors,

Fig. 1 This figure illustrates the application of oxygen-plasma treatment as an integrated or post-biochar-production processing technology to create biochar product with higher cation — exchange capacity

including those mentioned above, the biochar is typically exposed to the ionized oxygen for at least 0.1-5 min, with times as long as 60 min possible. Though the biochar is typically plasma treated within a temperature range of about 15-30°C, a lower temperature (e. g., less than 15°C) or a higher temperature (e. g., between 30 and 100°C) may be used. Generally, an oxygen plasma process is conducted as a combustionless process, i. e., without producing oxide gases of combustion.

Another possible method is the treatment of a biochar source with one or more oxygenating compounds (typically, oxygen in the form of air) at a temperature at which the oxygenating compound is reactive enough to impart oxygen-containing cation-exchanging groups to the biomass, i. e., at a suitably reactive temperature, wherein the amount of the oxygenating compound and/or time of reaction is appro­priately adjusted such that the biochar acquires the cation-exchanging groups in an incomplete combustion process. The reaction can be conducted as a combustionless process. Highly reactive oxygenating compounds can typically function effectively at room temperature (e. g., 15-30°C) or even lower temperatures (e. g., less than 15°C). Moderately reactive oxygenating compounds (e. g., oxygen) can typically function effectively within a temperature range of 100-950°C. Longer reaction times generally yield a more oxygenated biochar whereas shorter reaction times generally yield a less oxygenated biochar. Therefore, a moderately reactive or substantially unreactive oxygenating compound may effectively oxygenate biochar by use of a temperature of or less than 100°C if a sufficient period of time is used, ranging from at least 12 h to as long as 3 months.

Alternatively, an incomplete combustion process is attained by limiting the amount of oxygenating compound to an amount less than that required for complete combustion of the biochar source. A permissible amount of oxygenating compound less than required for complete combustion can be determined based on the amount of carbon contained in the biochar source, from which an amount of oxygenating compound less than required for complete combustion is calculated. The amount of

Fig. 2 This figure illustrates a process of injecting a calculated/limited amount of oxygen (O2) into the biomass-pyrolysis process to achieve partial oxidation of product biochar for enhanced cation — exchange capacity

carbon contained in the biochar sample can be determined by either an accurate measurement (e. g., by elemental analysis) or by an approximation (e. g., by weighing, and assuming nearly all of the weight to be from carbon materials). The amount of oxy­genating compound is preferably no more than about 20-30% of the moles of oxygenating compound required for complete combustion of the biochar, and, preferably, in the range of 0.1-15%.

The oxygenating compound can be reacted with biochar in a closed system in order to ensure that the intended amount of oxygenating compound as measured is reacted with the biochar. When an oxygenating solid or liquid oxygenating compound is used, the solid or liquid can be weighed into the closed container along with the biochar source and the contents homogeneously mixed or blended under conditions suitable for oxygenation of the biochar to take place. For example, the temperature of the mixed reactants in the container can be raised along with proper agitation until the solid or liquid becomes suitably vaporized in order to promote uniform reaction with the biochar. When an oxygenating gas is used, a selected volume of the gas corresponding to a calculated weight or moles of the gas can be charged into the closed system along with the biochar source before raising the temperature for more efficient oxygenation of the biochar. This could be applied also as a part of the biomass-pyrolysis process by injecting the calculated, limited amount of oxygen or air into the reactor system as illustrated in Fig. 2 . Alternatively, the process is simplified to opening the closed container containing an amount of biochar to cause the container to fil l with air, and then closed before proceeding with the heating process, thereby resulting in the addition of a limited amount of air.

Particularly when air or an artificial oxygen-gas mixture is used, the reactants are typically placed in a heatable closed system (i. e., a thermally insulated chamber), such as an oven, kiln, or furnace. The heatable closed system can be any suitable system typically operated or assisted by, for example, a flame (e. g., from a natural gas source), electricity, or microwaves. The kiln can be any of the downdraft, updraft, cross draft, fluid bed, or rotating kilns. The heatable closed system can also be one configured to adjust the moisture level of the biochar. The moisture level can be suitably adjusted to a humidity level anywhere between 1 and 100%. The reactants are heated in the closed chamber to a temperature or temperature range between 100 and 550°C. Heating can advantageously be minimized or altogether dispensed with

Fig. 3 This figure illustrates a process of quick air O2 exposure and water quenching of hot biochar as a post-biochar-production processing means to create biochar product with higher cation-exchange capacity

by reacting still hot biochar (i. e., as rendered hot by a biomass-to-biochar production process) under the oxygenating conditions of the invention. In this case, the hot biochar should be at a temperature between 250 and 450°C, or around 400°C.

In another variant of the method, the biochar and one or more oxygenating reactants are reacted for a period of time necessary for substantially all of the oxygenating reactant in a closed container to be consumed. The conditions of temperature and/or time can also be selected such that only a portion of the oxygenating reactant in a closed container is consumed.

An incomplete combustion process is attained by conducting the oxygenation reaction in an open or closed container and rapidly quenching the reaction. As illus­trated in Fig. 3 2 the reaction can be quenched by contacting the reacting biochar with an excessive amount of water and/or an inert substance, preferably when the biochar material is still hot, e. g., at a temperature of 150-450°C, as produced from a biomass-to-biochar process. The inert substance can be carbon dioxide or a form of biomass (e. g., soil, plant-material, or the like). The water and/or inert substance should preferably cover all of the reacting biochar, or alternatively, function as a bulk surface shield of the biochar, with the result that the oxygenating process (including a combustion process, if occurring) is immediately halted due to restricted access of the oxygenating compound to the biochar. If an elevated temperature is being used in the oxygenation process, the quenching step also typically has the effect of rapidly reducing the temperature of the biochar. A quenching step can alternatively be practiced by rapid sealing of an open container in which air- combustion of biochar is taking place.

The method described here can also include one or more preliminary steps for producing biochar (i. e., the biochar source or “produced biochar”) from biomass before the biochar is oxygenated. The biomass-to-biochar process can be conducted within any suitable time frame before the produced biochar is oxygenated.

A biomass-to-biochar process can be conducted in a nonintegrated manner with the biochar oxygenation process. In the nonintegrated process, biochar produced by a biomass-to-biochar process is transported to a separate location where the biochar oxygenation process is conducted. The transport process generally results in the cooling of the biochar to ambient temperature conditions (e. g., 15-30°C) before oxygenation occurs. Typically, the produced biochar is packaged and/or stored in the nonintegrated process before oxygenation of the biochar.

A biomass-to-biochar process can be conducted in an integrated manner with a biochar oxygenation process. In the integrated process, biochar produced by a biomass-to-biochar process is oxygenated in situ without first being cooled to ambient temperature. For example, freshly produced biochar can have a temperature in the range of 50-450°C, before it is oxygenated. If desired, the freshly produced biochar can be subjected to additional heating to elevate and/or maintain its temperature before the oxygenation step.

The biochar oxygenation process can be integrated with a biomass-to-fuel process, such as a low temperature or high temperature pyrolysis process. In such processes, typically about 40-60% of the biomass carbon is converted into biochar while the remaining 40-60% of the carbon is converted to fuel, such as H2 and/or bio-oils. Since it has been found that lower temperature pyrolysis processes generally yield a biochar material with better fertilizer retention properties, the biochar oxygen­ation process is best integrated with a biomass pyrolysis process conducted at temperatures of 350-450°C.

The integrated process can be configured as a batch process wherein separate batches of produced biochar are oxygenated at different times. The integrated process can also be configured as a continuous process wherein which biochar produced by the biomass-to-biochar process is continuously subjected to an oxygen­ation process as it is produced. For example, produced biochar can be continuously transported either manually or by an automated conveyor mechanism through a biochar oxygenation zone. The automated conveyor mechanism can be a conveyor belt, gravity-fed mechanism, or air pressure mechanism.

The technology concept is based on the utility of an oxygenated biochar having a particular, exceptional, or optimal set of characteristics, exemplified in the oxygen- to-carbon molar ratio, CEC, surface area, composition, and uniformity in these and other characteristics. The methods described above are particularly suitable for producing these types of biochars.

Dependent upon the reaction conditions, the CEC of the oxygenated biochar is at least moderate, that is, between 50 and 240 mmol kg-1, while preferably the CEC of the oxygenated biochar could be atypically or exceptionally high, in the range of 250-850 mmol kg-1. The CEC value should be substantially uniform throughout the biochar material.

The density of oxygen-containing cation-exchanging groups is typically propor­tional to the measured oxygen-to-carbon molar ratio of the biochar, wherein the higher the oxygen-to-carbon molar ratio, the greater the density of cation-exchanging groups in the biochar. The oxygen-to-carbon molar ratio of the oxygenated biochar is preferably adjusted within a range of 0.1:1-0.4:1 that is substantially uniform throughout the biochar material.

The oxygenated biochar should have a suitable specific surface area (SSA), as commonly determined by BET analysis, typically within the range 10-65 m2 g-1 and suitable pore size up to 100,000 nm. The oxygenated biochar has a suitable charge density in the range of 1-75 mmol m-2. It can have any suitable carbon, nitro­gen, oxygen, hydrogen, phosphorous, calcium, sulfur, ash, and volatile matter content. The carbon content is typically 20-95 mol%, the nitrogen content 0.1-8.0 mol%, the oxygen content 1-30 mol%, and the hydrogen content 1-30 mol%. The phos­phorus and calcium content can range from 5 to 25,000 mg kg-1 while the sulfur content is in the range of 50-2,000 ppm. The ash content ranges from 1 to 70% and the volatile matter content 1-40%.

The particle size of the oxygenated biochar should range between 50 and

5.0 pm. For certain applications for which it is important to ensure that the biochar materials are resistant to becoming airborne, such as use in windy and/or desert areas, larger biochar particle sizes, such as 6,000-50,000 pm, or even up to

100.0 pm will be desirable. The biochar materials may also be in the form of an agglomeration, compaction, or fusion of biochar particles (e. g., pellets or cakes) for this type of application as well.

Soil-fertilizing compounds or materials can be mixed with the oxygenated biochar to enrich it. Suitable soil-fertilizing compounds include nitrogen-based (e. g., ammonium-based), carbonate-based (e. g., CaCO3), phosphate-based (e. g., the known phosphate minerals, such as in rock phosphate or triple superphosphate), and potassium-based (e. g., KCl). Typically, the added compounds or materials would include at least one nitrogen source, usually NH4+-containing compound or material. Some examples of such nitrogen-containing fertilizing compounds or materials are (NH4)2CO3, NH4HCO3, NH4NO3, (NH4)2SO4, (NH2)2CO, biuret, triazine-based materials (e. g., melamine or cyanuric acid), urea-formaldehyde resin, and polyamine or polyamide polymers. Organic fertilizers can be mixed with the biochar. Some examples of organic fertilizer materials include peat moss, manure, insect material, seaweed, sewage, and guano. The biochar material can be combined with a fertilizer by any of a number of commonly used methods. For example, the biochar material can be saturated with a gas stream of hydrated ammonia to saturate the biochar material. Fertilizer compounds may also be applied as a coating to the biochar material, utilizing standard formulations for time-release of the fertilizer if desired.

Obviously, soil will be admixed with any of the various biochar compositions described above. The oxygenated biochar can potentially be used to improve soil of any type and composition and should be compatible with common soil components, such as clay, sand, silt, plant matter containing lignin and cellulose, peat, and humic substances.

In summary, this technology comprises a series of proposed methods to produce partially oxygenated biochar products with enhanced CEC properties for applica­tions as a soil amendment and carbon sequestration agent. A further detailed descrip­tion of the technology concept can be found in our recent US Patent application [10]. Application of biochar as soil amendment is potentially a revolutionary approach in achieving carbon sequestration globally at Gt C scales [11]. An improved biochar soil amendment material with enhanced CEC properties is beneficial to helping retain soil nutrients, reduce fertilizer run off, and ensure the associated soil and hydrologic environmental health. Therefore, this biochar CEC-enhancing technology could play a significant role in helping achieve this mission.

Acknowledgments This technology concept was funded through a subcontract from Iowa State University under USDA Grant No. 68-3A75-5-233 at Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for DOE under contract No. DE-AC05-00OR22725. MKK and ACB were funded in part by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, US Department of Energy.