Multi ea 4000

The physical properties of POM and MAOM and the SOC associated with Ca bridges (OC-Ca) and Fe oxides (OC-Fe) were determined. A fractionation technique was adopted to estimate the SOC stored in the POM and MAOM fractions. Air-dried soil was separated into light and high-density fractions with sodium polytungstate solution (1.60 g cm⁻³)^{66 (link)}; the high-density fractions were then wet sieved to collect POM (>53 μm) and MAOM (<53 μm)^{67 (link)}. Moreover, to determine OC-Ca and OC-Fe contents^{68 (link)}, the high-density fractions were extracted using 0.5 M Na₂SO₄ to release OC-Ca; the remaining residues were then extracted with citrate–bicarbonate–dithionite and sodium chloride for the treatment and control groups, respectively, and the differences in SOC content between the two groups were treated as the OC-Fe measurements. The SOC contents in these fractionations were ultimately determined using an elemental analyzer (Multi EA 4000, Analytik Jena, Germany) after inorganic C was removed with 1 M HCl.

Cation and anion concentrations were measured by inductively coupled plasma-optical emission spectrometry (CAP6300, Thermo, USA) and ion chromatography (ICS1100, Dionex, USA), respectively. Arsenic concentrations were determined using liquid chromatography-hydride generation-atomic fluorescence spectrometry (LC-HG-AFS, Haiguang AFS-9780, Beijing)33 (link). Thioarsenic species were identified using Q Exactive, a high resolution quadrupole orbitrap mass spectrometer (Thermo Scientific, Germany), by detecting the accurate mass and matching the isotope abundance. DOC of water samples were determined using a TOC analyzer (TOC-V_CPH, Shimadzu, Japan). As and Fe in the sediments were extracted by 1:1 aqua regia digestion in a water bath45 . Extracted Fe from sediments was determined by a 1,10-Phenanthroline-based assay: 10 mL extracted solutions were mixed with 5 mL acetate-sodium acetate buffer (pH = 4.6), 2.5 mL 1% hydroxylamine hydrochloride and 5 mL 0.1% 1,10-phenanthroline solution in a 50 mL volumetric flask. The mixture was brought up to a volume of 50 mL with deionized water and allowed to stand for 10 min. The absorbance of each solution at 510 nm was measured with a spectrophotometer (UV1750, Shimadzu, Japan). TOC of sediment samples was measured with a Macro elemental analyzer (Multi EA 4000, Analytik Jena, Germany) after inorganic carbon was digested using HCl.

NMR spectra were recorded on Bruker Avance III 500 (¹H: 500 MHz, ¹³C: 125 MHz) and Bruker Avance III HD 600 MHz (¹H: 600 MHz, ¹³C: 150 MHz) NMR instruments (Bruker Biospin, Ettlingen, Germany). Melting points were determined in open capillary tubes using a STUART SMP3 electric melting point apparatus (Bibby Sterilin Ltd., Stone, UK) and are uncorrected. Elemental analysis was performed with a multi EA 4000 (Analytik Jena, Jena, Germany) device.
Absorption spectra of all azo derivatives were recorded using a Jasco V-660 spectrometer (Jasco Corporation, Tokyo, Japan) in the 200–800 nm range, at room temperature, in a 1 cm quartz cuvette.
Fluorescence emission of the azo derivatives was measured on powder, using a OceanInsight XDH spectrometer (OceanInsight, Orlando, FL, USA) modular device coupled via optical fiber to a LED source emitting at 365 nm on reflection mode, and in solution using a Jasco V850 spectro-fluorimeter (Jasco Corporation, Tokyo, Japan). FTIR spectra were collected using a Tensor 37 Bruker equipment (Bruker Corporation, Billerica, MA, USA) instrument within the spectral range 400–4000 cm⁻¹, with 32 scans at a resolution of 4 cm⁻¹. Sample pellets were prepared by adding azo dye powder to KBr powder.

About 10–17 mg of the sediments were weighted into tin crucibles, a spatula tip of vanadium(V) oxide (Alpha Resources, Stevensville, MI, United States) was added as catalyzer and total C, total N, and total S were determined by an elemental analyzer (EuroEA, HEKAtech, Wegberg, Germany). For total inorganic carbon, 50–70 mg of sediment was treated with 40% orthophosphoric acid and analyzed with an elemental analyzer (multiEA 4000, Analytik Jena, Jena, Germany). Total organic carbon was calculated by subtracting total inorganic carbon from total carbon. Precision and trueness were checked with in-house standards [Mecklenburg Bay Sediment Standard (MBSS), Oder Bay Sediment Standard (OBSS)] and were <3.5% (Häusler et al., 2018 (link)).