Biochar is charcoal generated for intentional soil amendment by pyrolyzing sustainable biomass feedstocks. Properly “engineered” charcoals can increase the water holding and cation exchange capacities of soils, improving the ability of plants to survive under drought conditions and reducing fertilizer runoff into watersheds. Fertilizer runoff has become a serious problem, because as much as 70% of fertilizer applied to crop fields is leached into the groundwater or lost to streams and rivers, eventually leading to large hypoxic 'dead' zones in the world’s oceans (including the Gulf of Mexico).
Therefore, it is important to develop a theoretical framework that will allow us to evaluate the beneficial effects of soil amendment by biochar and, more specifically, to design biochars with optimal properties for each application. The environmental performance of biochars will depend on their ability to adsorb, retain and release water and nutrients. These biochar properties are controlled by their porosity and surface chemistry, which can vary widely depending on the composition of the biomass feedstocks and on the pyrolysis conditions employed during biochar production.
We present here the development of a model that uses experimentally determined biochar properties to predict their field performance in soil/biochar mixtures. Our model describes the dynamic adsorption/elution behavior of ammonium nitrate, a model fertilizer, in columns packed with mixtures of biochar and soil and perfused with aqueous solutions of the fertilizer. The chromatographic model accounts for all the important processes occurring in this system, including external mass transfer between the fluid phase and the solid particles and the all-important intraparticle diffusion and adsorption of the solute on the pore surface area of the sorbents. To our knowledge, this is the first chromatographic model that accounts explicitly for the presence of two solid phases with widely different pore structures and adsorption capacities. It also accounts for the complicated pore structure of biochars that consists of interconnected networks of large macropores, mesopores and micropores. The transient mass balances lead to a system of partial differential equations that is solved using orthogonal collocation on finite elements.
To provide the necessary data for our model, we characterized the chemical and pore structure of our biochars using a battery of analytical techniques. We also studied the adsorption kinetics of ammonium nitrate on biochar and soil particles in well-stirred batch reactors. In the case of biochars, we observed a very slow approach to equilibrium, which means that adsorption on these microporous sorbents is controlled by the slow rates of intraparticle diffusion. A parametric analysis was then carried out to determine how the transient behavior of the biochar/soil amendment system will be influenced by important operating parameters such as the solute concentration, the biochar/soil weight ratio, the flow rate of the aqueous solution, the pore structure of the biochar and its cation exchange capacity. The model is then used to analyze and interpret experimental adsorption/elution data from soil/biochar columns. The results show that the process is controlled by the adsorption kinetics of the solute and its intraparticle diffusion rates. This conclusion emphasizes the importance of carefully selecting the pyrolysis conditions employed for biochar preparation.