Ionpac

Pit mud moisture levels were detected via a gravimetric approach by drying samples for a minimum of 6 h at 105 °C. Pit mud pH was assessed using a pH meter and a 1:10 pit mud to boiled deionized water sample. Ammonium (NH₄⁺-N) present within pit mud was extracted using 10% (w/v) NaCl and measured with a UV–vis spectrophotometer (UV-9000, Bejing Puxi Instruments Co., Ltd). Total carbon levels in air-dried powdered pit mud samples were assessed with an Elementar instrument (Shanghai Yuanxi TOC-5000, China) in CHNS mode with the burning tube being heated to 1150 °C and the reducing tube being heated to 850 °C. Pure water was used to extract K⁺, PO₄³⁻, soluble Mg²⁺, and soluble Ca²⁺ from air-dried pit mud samples at a 1:10 (w/v) ratio, after which their concentrations were analyzed with an ion chromatograph (ICS5000+, ThermoFisher) equipped with conductivity detector (ICS-5000 + -DC) and a CS12 column (IonPac, ThermoFisher, 4 mm × 250 mm). A 25 µL injection volume was used for all analyses, with methane sulfonic acid (20 mM) as the carrier fluid at a 1 mL/min flow rate, and a constant column temperature of 30 °C.

Pit mud moisture levels were established by drying samples for 3 h at 115 °C. Pit mud pH values were established with a Mettler Toledo pH meter after diluting sample 1:4 (w/v) with dH₂O for 3 h. Pit mud ammonium (NH₄⁺-N) levels were established via extraction in 10% (w/v) NaCl at a 1:10 (w/v) ratio, after which concentrations were measured using a UV spectrophotometer. Acetic acid, butyric acid, and caproic acid were extracted using 15% methanol and quantified via gas chromatography (Agilent 7890, Santa Clara, CA, USA) as described previously [12 (link)]. Lactic acid (LA) levels were quantified via ultra-high-performance liquid chromatography (UPLC, Acquity I-class, Waters, Milford, MA, USA) as previously reported [1 ]. Levels of K⁺, PO₄³⁻, soluble Mg²⁺, and soluble Ca²⁺ in air-dried pit mud were measured via extracting samples with ddH₂O at a 1:10 (w/v) ratio, after which concentrations were measured as reported previously by Zhang et al. using an ion chromatograph (ICS5000⁺, Thermo Fisher, Waltham, MA, USA) equipped with a conductivity detector (ICS-5000⁺-DC) and a CS12 column (Ion Pac, Thermo Fisher, 4 mm × 250 mm) [4 (link)]. The utilized injection volume was 25 μL, with methane sulfonic acid (20 mM) as a carrier fluid at a 1 mL/min flow rate, with a column temperature of 30 °C. Humus levels were determined as detailed previously by Shen [13 ].

Nitrate release from SLC-systems was determined by placing each xerogel in a flask containing 100 mL of Britton–Robinson buffer solution at the selected pH and placed into the thermostatic bath at the conditions indicated above (150 rpm, 25 °C). At different fixed time intervals, two aliquots of the release medium were taken for further analysis by ionic chromatography. This procedure was repeated until a constant concentration was reached for each sample.
The concentration of nitrate ions was determined by ionic chromatography using a DX-120 Vertex Technics S.L. equipment (Barcelona, Spain). Separation was performed on an AS9-HC 4 × 250 mm Dionex IonPac analytical column. The mobile phase was a 9 mM solution of Na₂CO₃ and an ASRS ULTRA II-4 mm Dionex was used as suppressed column.