Humic acid sodium salt

Chemicals were supplied by commercial sources and used without further purification: ammonium hydroxide solution (NH₃ in H₂O, 25%, Riedel-de-Häen), chromium(III) (Cr³⁺ in HNO₃, 1000 mg/L, Inorganic Ventures), cobalt(II) chloride hexahydrate (CoCl₂.6H₂O, 98%, Panreac), ethanol (C₂H₅OH, > 99%, Panreac), humic acid sodium salt (technical grad, Sigma-Aldrich), iron(II) sulfate heptahydrate (FeSO₄.7H₂O, > 99%, Merk), nitric acid (HNO₃, puriss. p.a., 65%, Merck), potassium hydroxide (KOH, > 98%, Pronolab), potassium nitrate (KNO₃, > 99%, Merk), and sodium hydroxide (NaOH, > 98%, Pronolab).

Model identification experiments were performed using humic acid sodium salt (technical grade from Sigma-Aldrich) at a concentration of 20 mg/L. This concentration was high enough to produce severe fouling in a short time. The experiments were carried out between 2 and 5 h. The temperature of the feed tank was maintained at 25 °C. The applied pressure range to build the model was between 0.3 to 2.5 bar. During the experiments, the inlet and outlet pressure, the concentrate flow rate, and the permeate flow were sampled at a rate of 1 s. The solenoid valve opening percentage and pump rotational speed were the two variables manipulated by the control system during the fouling operating stage to meet the pressure and flow requirements. The control actions were carried out at a rate of 20 s. To recover the permeate flow, a CIP operation was performed after each model identification experiment using a 0.1 mol/L sodium hydroxide solution.

Nitric acid (ACS reagent, 70%) and sulfuric acid (ACS reagent, 95.0–98.0%) were purchased from Sigma-Aldrich (Saint Louis, MO, USA). The raw multi-wall carbon nanotubes (abbreviated as CNT, CM-95, 93–97%) were purchased from Hanwha-nanotech (Seoul, Republic of Korea). PES (Veradel^® 3000 P, Brussels, Belgium) with a molecular weight 62,000–64,000 g/mol, was obtained from Solvay Specialty Polymers. N-methyl-2-pyrrolidinone (NMP, anhydrous 99.5%) and polyvinylpyrrolidone (PVP) with an average molecular weight 10,000 g/mol, were purchased from Sigma-Aldrich. Humic acid sodium salt (HA, technical grade) was purchased from Sigma-Aldrich for filtration and antifouling performance tests.

In this work, a Humic Acid sodium salt with technical grade (CAS: 68131-04-4) was purchased from Sigma-Aldrich. A stock solution was prepared, as the foulant, by dissolving an amount of HA in deionized water. A standard concentration of (50 ppm) of Humic Acid was prepared by dissolving HA in DI. The working solutions of required concentration were prepared by sequential dilution of the standard solution. The control foulant concentrations were measured using a UV-visible spectrophotometer at λ_max = 600 nm at initial concentrations of 15–45 mg/L. Consequently, the fouling performance of the polyethylene membranes was tested in the pilot-scale DCMD system. The HA rejection was calculated based on the HA concentration obtained by the UV spectrophotometer. The initial HA concentration was measured by taking a sample from the feed before starting the DCMD experiment (the control), and the final HA concentration that is collected from the permeate is the measured value. The rejection values were calculated for different HA concentrations using the difference between the control and measured concentrations divided by the control adsorption.

Sodium arsenite, sodium arsenate dibasic heptahydrate (≥98%), methanol (Optima^TM for HPLC), tert-butanol (t-BuOH), acetonitrile (HPLC grade), o-phosphoric acid (85%, ACS grade), formic acid (88%), ammonium hydroxide (29.2%), and ferric chloride hexahydrate (FeCl₃·6H₂O, 98.8%) were purchased from Fisher Scientific (Waltham, MA, USA). Ferrous chloride tetrahydrate (FeCl₂·4H₂O, ≥99%), humic acid sodium salt (Lot#STBC5468V), coumarin (COU) (≥99%), potassium sorbate (≥99%), and 7-hydroxycoumarin (7-HC) (99%) were obtained from Sigma Aldrich (St. Louis, MO, USA). Furfuryl alcohol (FFA) and the tetrabutylammonium hydroxide (TBAH) (40 wt.%) were purchased from Acros Organic (Geel, Antwerp, Belgium). The 2,4,6-trimethylphenol (98%), was obtained from TCI (Portland, OR, USA). The malonic acid (reagent grade, 99.5%) was purchased from Alfa Aesar (Haverhill, MA, USA). Millipore water (MilliQ water, resistivity~18.0 MΩ cm⁻¹ at 25 °C) was used for sample and standard preparation unless otherwise indicated.