Nitric acid

AAPH (2,2′-azobis (2-amidinopropane)) (Acros Organics), sodium phosphate dibasic, sodium chloride, potassium dihydrogen phosphate, potassium sulphate, TRIS (ultrapure), glycine (proteomics grade), ortho-boric acid, and methanol (HPLC grade) were obtained from VWR International Eurolab S.L. (Barcelona, Spain). Trizma^® base, ABTS (2,2′-azinobis (3-ethylbenzothiazoline 6-sulfonic acid)), DTT (DL-Dithiothreitol), Trolox^® (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), fluorescein sodium salt, formic acid (reagent grade ≥ 95%), and diatomaceous earth (Hyflo^® Super Cel^®) were provided by Sigma-Aldrich (Steinheim, Germany). Sodium hydroxide, glacial acetic acid, and sulfuric acid were supplied by Fisher Scientific (Madrid, Spain). SDS (sodium dodecyl sulfate) and nitric acid (65% p/p) were purchased from Panreac (Barcelona, Spain). Bromophenol blue indicator (ACS reagent), acetonitrile (HPLC grade), trifluoroacetic acid, acetone, and glycerol were provided by Merck (Darmstadt, Germany). Absolute ethanol was obtained from J.T. Baker (Deventer, The Netherlands), Octadecyl C18 sorbent was obtained from Phenomenex (Madrid, Spain), and anhydrous magnesium sulfate (99.5% min powder) was obtained from Alfa Aesar (Karlsruhe, Germany). Deionized water with a resistivity of >18 MΩ/cm was obtained through a Milli-Q SP^® Reagent Water System (Millipore Corporation Bedford, MA, USA).

Reagents manufacturers: sodium nitrite, aluminum chloride, and sodium hydroxide, all extra pure (>99%), saline water (0.9% NaCl), and methanol were acquired from Merck (Merck, Darmstadt, Germany). Folin–Ciocalteu's reagent, 3,4,5-trihydroxybenzoic acid (gallic acid), acetic acid, both extra pure (>99%), and sodium hydroxide were purchased from Panreac (Panreac Química S.L.U., Barcelona, Spain). Sodium molybdate (99.5%) was obtained from Chem-Lab (Chem-Lab N.V., Zedelgem, Belgium). The compounds 2,2-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) diammonium salt (ABTS+), 2,2-diphenyl-1-picrylhidrazyl radical (DPPH), potassium phosphate, catechin, potassium persulfate, sodium acetate, 2,4,6-Tripyridyl-s-Triazine (TPTZ iron reagent), acetic acid, hydrochloric acid, and iron(III) chloride were obtained from Sigma-Aldrich (Sigma-Aldrich Produktions GmbH, Steinheim, Germany). Additionally, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) was purchased from Fluka Chemika (Fluka Chemika, Neu-Ulm, Switzerland). Ethanol, ether petroleum, potassium persulfate, sodium hydroxide, methanol, hydrochloric, formic, and nitric acid, acetonitrile, iron(III) chloride, sodium acetate, sodium carbonate, sulphuric acid (98%), and potassium hydroxide (98%) were acquired from Panreac (Panreac Química S.L.U., Barcelona, Spain).

The materials used in this study were as follows: graphite powder, <20 μm, synthetic, Aldrich; sulfuric acid, H₂SO₄, 96.5 %, Baker; fuming nitric acid, HNO₃, ≧99.5 %, Sigma-Aldrich; potassium permanganate, KMnO₄, Baker; PDDA, 35 % (average M_w < 100,000), Aldrich; hydrogen peroxide, H₂O_2(aq), 35 %, Acros; hydrochloric acid, HCl_(aq), 37 %, Scharlau; sodium citrate dehydrate, Na₃Ct·2H₂O, ≧99.5, Sigma-Aldrich; hydrogen tetrachloroaurate(III) trihydrate, HAuCl₄·3H₂O, 99 %, Sigma-Aldrich; nitric acid, HNO_3(aq), 69 %, Panreac; silicon Oil, Choneye Pure Chemical; Luria-Bertani (LB broth), Difco™ (Agar Bacteriological), Oxoid; and adenine, C₅H₅N₅, ≧99 %, Sigma.

Cylinders of commercially pure ASTM grade II Ti (Manfredi, S. Secondo di Pinerolo, Italy) was used. Each cylinder was cut into disks with a thickness of 2 mm, followed by a metallographic polishing protocol. The samples were polished using silicon carbide papers from grade 500, 800, 1200, 2000, to 4000 grit. They were then ultrapolished with a sequence of 1 to 0.3 to 0.05 μm alumina particles in a Beta grinder/polisher (Buehler, Munich, Germany). Next, samples were cleaned by immersion in an ultrasonic bath (Selecta, Barcelona, Spain), first in 50% ethanol (vol) (10 min) and then in distilled water (10 min).
The Ti disks were divided into 6 groups for different etching times: 0 min (no etching), 2 min, 3 min, 5 min, 7 min, and 10 min. Etching was performed by immersing samples in a solution of 10% (v/v) HF acid (Panreac, Barcelona, Spain) under magnetic agitation, with the Ti surface at right angles to the direction of the solution. Next, the etched samples were gently washed with distilled water and passivated with 30% (v/v) nitric acid (Panreac, Barcelona, Spain) for 3 min and were again ultrasonicated in 50% ethanol for 10 min and distilled water for 10 min, followed by drying at 37°C for 1 h. The sample was weighed before and after etching to calculate the percentage mass loss.

The IL precursors trihexyltetradecylphosphonium chloride (95%) and 3-hydroxy-2-naphthoic acid (98%) were purchased from Sigma-Aldrich (USA), methyl-trioctylphosphonium methylcarbonate (purum, 50% in methanol) and methyl-trioctylammonium methylcarbonate (purum, 50% in methanol) from Proionic (Austria). For synthesis and sample preparations, potassium hydroxide (p.a., 85%) from Merck (Germany), methanol (HPLC grade, 99.9%), dichloromethane (HPLC grade, 99.9%), sodium chloride (99%), nitric acid (p.a., 65%), and sodium hydroxide (98%) from Panreac (Spain), and humic acid sodium salt (technical grade) from Sigma-Aldrich were used. Standard solutions of metals Ag, Cd, Co, Cu, Mn, Ni, and Pb 1,000 mg L⁻¹ in 2–4% (w/w) HNO₃, purchased from Sigma-Aldrich, were used for atomic absorption spectroscopy measurements and to prepare feed solutions. Ultra-pure water of resistivity <18.2 MΩ cm was obtained from a Millipore Milli-Q Academic apparatus (Merck Millipore, USA).

Hydrogen peroxide (H₂O₂, 30% w/v), ferrous sulfate heptahydrate (FeSO₄.7H₂O, 99.0% purity), sodium sulfite (Na₂SO₃, 98.0% purity), sodium hydroxide (NaOH, 99.0% purity), sulfuric acid (H₂SO₄, 97–98% purity) and nitric acid (HNO₃, 65% purity) were purchased from Panreac. Pure liquid toluene (99% purity) (chemical formula C₇H₈, molar mass = 92.14 g mol⁻¹) and di-sodium hydrogen phosphate anhydrous (99% purity) were purchased from VWR Chemicals.
A commercial activated carbon, Norit GAC 1240 AF, was used as the initial material to prepare all modified activated carbons and AC-Fe catalysts. Melamine (C₃H₆N₆, ≥ 99% purity) and iron (III) nitrate nonahydrate (FeH₁₈N₃O₁₈, 99% purity) were obtained from Fluka.