Nutrient sorption potential of treated and untreated hydrochars and biochars derived from various waste feedstocks

Biochars have traditionally been associated with soil amendment but are also useful in a number of sectors as they show potential to be cost-effective, multi-functional products particularly if they are produced from waste biomass. Current research is geared towards enhancing char agronomic value via physical, chemical and/or biological means although further studies are still required to gain a better understanding of the parameters which can be optimized to produce chars with specific functionality. This research set out to evaluate the potential for hydrochars and biochars derived from herbaceous and treated municipal waste to be used for nitrogen and phosphorus recovery from simulated wastewater, in addition to ammonia gas emission reduction during co-composting. This study also focused on providing more insight on some of the factors influencing hydrochar and biochar performance in nutrient-rich environments and investigating the potential for modifying char characteristics for enhanced nutrient recovery. Consequently, analysis of the physicochemical properties of hydrochars and biochars produced from paprika waste from a greenhouse, the treated organic fraction of municipal waste, greenwaste and pig manure has been performed. Comparisons are also made with relatively low-contaminant hydrochars and biochars derived from bark-free holm oak wood. Processing parameters include hydrothermal carbonization at 250°C for 60 min, slow pyrolysis at 400–700°C and gasification at 600–750°C over 30–60 min residence times. As oak and paprika waste chars possess carbon contents >50%, these have been categorised as Class 1 biochars in accordance with the international biochar initiative product specifications, while hydrochars and slow pyrolysis biochars derived from municipal waste, presscake, and greenwaste are ranked as Class 2–3 chars. in spite of differences in biomass inorganic content, the various feedstocks decompose into chars in a similar manner. Char morphological properties are observed to be more dependent on processing temperature and reactor system than to feedstock property, based on the substantial differences in surface area of holm oak biochars produced using three different reactor types. However, from batch sorption tests with synthetic wastewater, char surface area and porosity are of less importance than char oxygen and inorganic mineral contents in terms of ammonium and phosphate sorption, respectively. Overall however, all chars demonstrate similarly low capacities for ammonium and phosphate sorption (up to 14.6% and 7%, respectively). Conversely, in terms of ammonia removal, two of the hydrochars selected for further study are shown to possess higher ammonia emission reduction capacities relative to their biochar counterparts in 17-day laboratory co-composting trials. These differences are likely attributable to the acidic functional groups present in the hydrochars. While both oak and greenhouse waste hydrochars demonstrated higher levels of inorganic nitrogen (ammonium and nitrate) mineralization relative to their biochar counterparts, mineralization and carbon dioxide evolution was more prominent in the latter hydrochar. These findings are in agreement with previous studies in the literature, which have shown that hydrochars possess more mineralizable carbon and nitrogen species than biochars. Following from an understanding of the respective effects of char acid oxygen groups and inorganic content on char ammonium and phosphate sorption capacities, attempts have been made to enhance these properties via mild chemical activation of biomass or char samples. Results show that modest increases in both ammonium and ammonia sorption capacity of the chars can be achieved following acid treatment, while phosphate sorption can be enhanced from low levels (2.1–3.6%) to relatively high levels (66.4–70.3%) by impregnation with magnesium. Various treatments will understandably produce different effects on the different hydrochars. This is evident in the case of greenhouse waste, which experiences a considerable increase in ammonia sorption capacity following potassium hydroxide treatment of greenhouse waste 250°C hydrochar and 400°C biochar, from 3.3% to 44.1% in the latter char while the effect is less pronounced following sulphuric acid treatment. Overall, findings from this study suggest that it is possible to enhance waste-derived char capacity for ammonia / ammonium and phosphate recovery by treatment of chars or char precursors (raw feedstock) via mild chemical activation processes.