# Analysis of Influencing Characteristics of Biochars for Ammonium Adsorption
## 1 Introduction  
The ammonium ion (NH4+) is one of the most concentrated species of inorganic nitrogen in wastewater, originating from human urine (4 g/L [1]), fertilizer runoff (1 kg/ha+year [2]) and agricultural slurries (1.7-2.5 g/L [3]). As ammonium is a main ingredient used to produce many fertilizers, its abundance in wastewater streams calls for recovery attempts to be made to decrease the proportion of ammonium produced by the energy and resource intensive Haber -Bosch process. Nitrogen recovery from waste streams is a popular topic, with a race in research to find technology that allows for efficient nitrogen recovery with low energy requirements and minimal material costs [4]. Popular techniques include physical removal (using filters, membranes, adsorbents), biological (transformation using metabolic pathways of bacteria or algae) and chemical separation (precipitation from solution through the formation of struvitve). One technique with low energy requirements and simple implementation is adsorption, where ammonium ions can be attached to material by means of bonding to the removal surface. Adsorption mechanisms may include electrostatic attraction, ion exchange, physical entrapment, chemical bonding with surface functional groups, and precipitation. Biochar, the solid product from pyrolysis, has been targeted as an adsorbent of interest as it allows for the valorization of waste products like agricultural, municipal, and industrial wastes, creating a circular economy. Biochar can be used for carbon sequestration and is estimated to be able to reduce net emissions of carbon dioxide, methane and nitrous oxide by12% per year when used in soil [5]. It has variable surface area, surface functional groups, charge, etc. that are dependent on feedstock origins and characteristics, as well as process conditions. Pyrolysis temperature has been linked to pore volume, surface area, surface pH, cation exchange capacity, ash, carbon and volatile content among other parameters [6]. Using biochar as an adsorbent can allow for the production of either liquid or solid fertilizers. To produce a liquid fertilizer, ammonium can be bound to the surface and then desorbed and concentrated using additional methods. If the ammonium is bound strongly to the biochar surface, the loaded biochar can be used as a solid fertilizer, as the sorbed ammonium has been shown to be bio-available [7]. Aluminum-silicate materials such as zeolite have shown to be useful in full scale wastewater applications for ammonium recovery since the 1990s [8] due to their inherent affinity for ammonium, adsorption capacity, and ability to desorb and be regenerated into reusable adsorbents. Biochar has shown to have much less efficient desorption when compared to zeolite [9]; therefore, it may be used as a solid fertilizer after adsorption instead. As biochar is such a diverse material, with properties depending on material, process conditions and treatments, the range of ammonium adsorptions recorded is vast. Even so, the potential for circular economy connection agriculture and wastewater treatment is interesting [10]. Some individual studies have reported ammonium adsorption as high as 137.3 mgNH\({}_{4}\)+/g biochar [11], and others have reported less than 1 mgNH\({}_{4}\)+/g biochar [12], but most have not been corroborated with more than one study using the exact same conditions and materials. Many studies have tried to vary biochar processing conditions using different materials to optimize adsorption of ammonium, but the analysis of the physical and chemical characteristics chosen in literature is not consistent. It is therefore difficult to compare different results from different studies, and to draw conclusions about what specific characteristics are responsible for increasing or decreasing ammonium adsorption capacity. This article provides an overview of experimental studies using biochar to adsorb ammonium, including an analysis of physical and chemical characteristics to determine the most important factors that influence adsorption capacity.