Using farmland to digest biogas slurry is an effective measure to overcome the bottleneck of sewage treatment in livestock and poultry farms. However, there is limited research on the soil adsorption characteristics of biogas slurry ammonium nitrogen (NH4+-N). In addition, the maximum adsorption capacity (Qm) of farm soil is unclear. In this study, three typical farmland tillage layer soils (silty loam, loam, and sandy loam) were used to analyze adsorption characteristics through adsorption kinetics experiments (adsorption for 0.25, 0.5, 1, 2, 4, 6, 12, 18, or 24 h with NH4+-N concentrations of 42.90 mg/L) and thermodynamic experiments (adsorption for 3 days with NH4+-N concentrations of 54.25, 88.66, 105.85, 133.71, 178.80, 273.54, and 542.87 mg/L). The Qm value was fitted by models, and its relationship with soil properties was discussed. The results showed the following: 1) the adsorption of biogas slurry NH4+-N by the three types of soils was a composite kinetic process that comprised two stages of rapid and slow reactions. Rapid adsorption predominantly occurred within 0–1 h, and the adsorption capacity accounted for 35.24%–43.55% of the total adsorption. The ExpAssoc equation produced a good fit for the adsorption kinetic behavior in the three soil types. 2) The equilibrium adsorption could be described by the Langmuir equation, the Freundlich equation, the PlPlatt model, and the Langevin model isotherm, among which the Langevin model had the best fit, with a coefficient of determination R2 close to 1. The theoretical saturated Qm fitting results of NH4+-N were 1038.41–1372.44 mg/kg in silty loam, 840.85–1157.60 mg/kg in loam, and 412.33–481.85 mg/kg in sandy loam. The optimal values were 1108.55, 874.86, and 448.35 mg/kg for silty loam, loam, and sandy loam, respectively. 3) The Qm value was significantly positively correlated with soil organic matter, total nitrogen, available phosphorus, available potassium, cation exchange capacity, and particle content of 0.02–0.002 mm (p < 0.01), but significantly negatively correlated with soil pH (p < 0.05). This study can provide a reference for the safe application of biogas slurry on farmland.
Water environment safety is the focus of engineering measures to eliminate liquid digestate in farmland. It is of great significance to study the aging characteristics of soil absorbing and fate of liquid digestate ammonium nitrogen (NH4+-N) to realize safe and efficient disposal. In this paper, simulation experiments of digesting NH4+-N (with application of 0, 120, 180, and 300 kg/hm2) by static soil column are carried out to study disposal efficiency, migration and transformation characteristics, and fate proportion of NH4+-N in saturated water content soil. The result showed that after 3 days of application, the overlying water NH4+-N concentration decreased by 63.5–80.7%, and the reduction rate of total NH4+-N was 65.8–82.3%. After 4 days, the NH4+-N concentration of pore water in the 0–10 cm soil layer reached the peak value. After 7 days, the NH4+-N concentration adsorbed by the 0–10 cm soil layer reached the peak value. After 15 days, the overlying water NH4+-N concentration decreased by 97.0–98.7%, the reduction rate was 97.9–99.2%, and the proportion of NH4+-N absorbed in the 0–10 cm soil layer accounted for 63.5–76.3%. The disposal is mainly based on soil sorption and pore water migration. A duration of 0–3 days is the rapid disposal period, and 15 days is the completion period of safe digestion.
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