# Adsorption of Ammonium, Nitrate, and Phosphate on Hydrochars and Biochars
## 1 Introduction  
The nomenclature employed to refer to the residual material derived from the slow pyrolysis of biowaste, which is primarily composed of carbon, is biochar (BC). Conversely, the residual material obtained from the hydrothermal carbonisation (HTC) process, also called wet pyrolysis, is referred to as hydrochar (HC). Both of the aforementioned products have garnered significant attention owing to their potential applications in a variety of engineering and industrial fields [1]. The production of BC involves a pyrolysis process that necessitates an extra energy input to dry the feedstock. Conversely, the production of HC enables the utilisation of wet feedstock that does not require drying during theprocess. The biomass undergoes a high-temperature treatment in the presence of water, which serves as a catalyst [2]. BC and HC exhibit promising characteristics and structures that make them suitable for utilisation as soil enhancers. These features include the capacity to enhance crop yield, facilitate the filtration of percolating soil water, and assimilate atmospheric carbon in soil [3]. The variances in the characteristics and functions of BC that have been observed are subject to the influence of various process parameters [4; 5; 6; 7; 8]. These parameters include the feedstock, processing temperature, heating rate, and residence time. The synthesis of HC can be achieved through the utilisation of water as the reaction medium at a lower temperature, resulting in a more cost-effective process compared to pyrolysis. The reaction conditions commonly observed in the literature include temperatures ranging from 180 to 260 \({}^{\circ}\)C, autogenetic pressures ranging from 2 to 6 MPa, and reaction times ranging from 30 to 240 min [9]. The surface of HC exhibits a higher concentration of functional groups containing oxygen and molecules that have undergone aromatisation. The incorporation of functional groups into the molecule enhances its hydrophilicity, thereby rendering it suitable for augmenting the water retention capacity of soil [10]. Distinguishable variations exist in the physical and chemical characteristics of BCs and HCs. The study by [PERSON] and colleagues [11] revealed that BC and HC exhibit adsorption capacities ranging from around 0 to 30 mg\(\cdot\)g\({}^{-1}\) and 105 to 146 mg\(\cdot\)g\({}^{-1}\) for phosphate and ammonium, respectively. This observed phenomenon can be attributed primarily to the unique physicochemical characteristics of the subject in question. The investigation of the properties and interaction mechanisms of BC and HC adsorption is crucial for the utilisation of soil fertilisers in order to mitigate potential environmental consequences. Eutrophication is a phenomenon that occurs in estuaries and coastal waters as a result of an overabundance of nitrogen and phosphate. This process leads to an escalation in the growth of plant and toxic algae [12]. The application of synthetic fertilisers has been found to have a positive correlation with the release of ammonium, nitrate, and phosphorus into water bodies from agricultural land [13; 14]. Research [15; 16] findings indicate that BC and HC have the ability to impede the uptake of nitrogen and phosphorus by the soil. The utilisation of sugarcane bagasse BC has been observed to enhance the retention time of nitrate in the vicinity of plant roots, thereby facilitating its uptake by the plants [17]. The optimal conditions were identified for ammonium adsorption on bamboo charcoal, which resulted in the removal of up to 82.4% of ammonium from an aqueous solution [18]. As per existing research, it has been observed that BC has the ability to solubilise phosphate. It has also been noted that modifying the concentration of BC present in soil can lead to improved agronomic performance [19]. As per the findings of a previous study [20], it can be inferred that HC exhibits potential as an adsorbent for the purpose of removing and recovering phosphorus from wastewater.