Fe20k

The pH value was measured by a pH meter (METTLER TOLEDO, FE20K). The content of BOD₅ was measured by incubation method. The content of total nitrogen (TN) and total phosphorus (TP) were analyzed using alkaline potassium persulfate digestion-UV spectrophotometer and molybdenum blue spectrophotometric (Carranzo 2012 ). Dissolved organic carbon (DOC) in water samples were measured using UV-per-sulfate technique and infrared carbon dioxide analyzer (Phoenix 8000), and calibrated with potassium hydrogen phthalate.

A 1.0% (w/w) LGG solution and 0.15% (w/w) TP solution were prepared by dissolving them in deionized water and stirring at 90 °C to ensure complete powder hydration and dissolution. The pH of the system was adjusted to 3, 4, 5, 6, 7, 8, and 9 with 1.0 M NaOH or 1.0 M HCl solution. A pH meter (FE20K, Mettler Toledo, Leicester, UK) was used to measure the pH of the solution. Then, 0.15 mL of 2 M Ca²⁺ solution was added to the compound gel. Finally, the compound gel was placed in a plastic casing while it was still hot and stored in a refrigerator at 4 °C for 24 h after cooling.

In the laboratory, roots and stones were picked out using forceps and soils were air dried and passed through a 2-mm mesh sieve. Soil organic carbon (SOC) was measured by wet oxidation with dichromate redox colorimetric method, with which carbonates are not determined37 . Soil pH (1:2.5 soil/water ratio) was measured with a pH meter (FE20K, Mettler-Toledo, Switzerland). Soil N was analyzed using an elemental analyzer (EA 3000; EuroVector, Italy). Soil total P was determined by acid digestion with a H₂SO₄ + HClO₄ solution37 . Soil texture was determined using a particle size analyzer (Mastersizer, 2000, Malvern, UK). Exchangeable calcium (Ca), magnesium (Mg), potassium (K) and sodium (Na) were displaced via compulsive exchange in 1 mol L⁻¹ ammonium acetate at pH 7.0 and analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES)38 . Final values of the above variables were reported on a dry soil basis, where dry soil denotes soil was dried to constant weight at 105 °C for 24 h.

The pH values of gas-free, N₂-satuarated, and CO₂-saturated electrolyte solutions (0.1 M KCl) were determined by an electronic pH meter (METTLER TOLEDO FE-20K). To determine the pH value of the CO₂-saturated electrolyte solution at ambient temperature, CO₂ was bubbled into 20 mL solution in a beaker (25 mL/min) under stirring. The pH value of the solution was monitored at different times, and the value was recorded when the value became unchanged with time.

The in vitro degradability of the samples (Φ10 × mm) was assessed by measuring the weight loss ratio of the samples (i.e., MP0, MP4, MP6 and MP8) in phosphate buffered saline (PBS)^{35 (link)}. The samples were weighed (W_i) and immersed in PBS (pH = 7.4) in sealed polyethylene bottles with a solid/liquid ratio of 0.2 g/20 mL, and the bottles were placed in an orbital shaker at 37 °C with constant shaking for 12 weeks. The PBS solution was refreshed once a week. At different time points (3, 7, 14, 21, 28, 42, 56, 70 and 84 days), the samples were removed from the solution and rinsed gently with deionized water. Then, the samples were dried at 50 °C for 24 hours, and the samples were weighed (W_f). The weight loss of the samples in PBS was calculated according to the following equation: $$\mathrm{Weight}\mathrm{loss}(\%)=\left({\mathrm{W}}_{\mathrm{i}}-{\mathrm{W}}_{\mathrm{f}}\right)/{\mathrm{W}}_{\mathrm{i}}\times 100$$
The pH value of the PBS solution containing the samples was measured using a pH meter (FE20K, Mettler Toledo, Switzerland) at different time points (1, 2, 4, 12 and 21 days), and the surface morphology the samples was observed using SEM after soaking into the PBS solution for 3 weeks.

Strain H at the logarithmic growth phase was collected by centrifugation. The strain was then resuspended using physiological saline water. The resulting suspension was used as degradation inoculation solution for all the following experiments. Effects of environmental conditions for the growth of strain H and heptachlor degradation were investigated. In each of our experiments, a 250 mL Erlenmeyer flask was used which contains 200 mL sterilized MSM containing 300 μg L⁻¹ heptachlor and 20% of inoculation volume with a biomass content of 1.2 × 10⁸ cells per mL, and the degradation time was 140 h. Tests on the effect of temperature were first examined at five levels, 20, 25, 30, 35 and 40 °C. The temperature is automatically set and adjusted by the constant-temperature rotary shaking incubator (BS-1E, China), at pH 7. These were followed by tests on the effect of pH at six levels from pH 5.2 to 9.3. The pH level is adjusted with 10% NaOH solution and 10% HCl solution. The pH level is measured by the pH meter (FE20K, METTLER TOLEDO, Switzerland) at the identified optimal temperature. Experiments on heptachlor degradation were carried at the optimal pH and temperature in 250 mL Erlenmeyer flasks at 120 rpm. Each flask contained 20% of inoculation volume (1.2 × 10⁸ cells per mL) and 200 mL MSM with heptachlor initial concentration varying from 100 to 500 μg L⁻¹.