Phenanthrene was purchased from Acros Organics (New
Jersey, USA) with a labeled purity of > 99% and used
as received. Oxalic acid (pKa1 = 1.25 and pKa2 = 4.27)
and citric acid (pKa1 = 3.13, pKa2 = 4.76, and pKa3 =
6.40) of reagent grade in sodium salt were from Beijing
Chemical Reagent Company, China. Amberlite XAD-2
resin (Supelco, Bellefonte, USA) has a particle size of 20–
60 mesh, a surface area of 300 m2/g and a pore diameter
of 90 nm. Before use, the XAD-2 beads were rinsed in
succession with methanol, a freshly prepared solution of
0.01 mol/L CaCl2 and 200 mg/L NaN3 in Milli-Q treated
water (10 times for each solution, 10 mL/g XAD). The
equilibrated XAD-2 resin was refrigerated as slurry in a
glass flask with an excess of 0.01 mol/L CaCl2 solution to
prevent dehydration. NaN3 was added to inhibit microbial
activity in the aqueous solution and microbial growth on
the XAD-2 resin beads. CaCl2 solution was decanted off to
leave the resin as a damp solid before use.