Electrochemical Oxidation of PAHs in Water from Harbor Sediment Purification

Sediments of harbors are regularly dredged for various reasons; maintenance of navigational depths, recovery of recreational locations, and even environmental recovery. In the past, the harbor sediment have been dumped at sea, however, environmental regulations now, in many cases, prohibit this due to contamination by PAH, heavy metals, TBT etc. In Denmark, contaminated harbor sediment is pumped ashore to inland lakes or upland sites where treatment of the runoff water is required before discharge to the recipient. In this study, electrochemical oxidation (EO) has been investigated as a method for treatment of the discharge water addressing primarily polycyclic aromatic hydrocarbons (PAHs). PAHs are by-products of incomplete combustion of organic materials with recalcitrant and strong mutagenic/carcinogenic properties, due to their benzene analogue structures. PAHs are hydrophobic compounds and their persistence in the environment is mainly due to their low water solubility.

The experimental study was performed in laboratory scale with volumes of water from 3 to 10 L in a batch recirculation experimental setup at constant temperature with a commercial one-compartment cell of tubular design with Ti/Pt90-Ir10 anode (60 cm2) and SS 316 cathode operated at galvanostatic conditions. The EO of naphthalene, fluoranthene, and pyrene was investigated in model solutions, in order to study the reaction kinetics and the influence of variations in experimental parameters such as current density, electrolyte composition, and electrolyte concentrations on the rate of oxidation, followed by a proof-of-concept study with the actual discharge water from a dump of contaminated sediment.

In the model solutions, all three of the subjected PAHs were degraded during the electrochemical treatment, and all of the conducted experiments confirmed that the removal rate of the two-ring structured naphthalene was significantly faster compared to the four-ring structured compounds fluoranthene and pyrene. In a Na2SO4 inert electrolyte, all three PAHs were degraded by direct electrochemical oxidation at the anode surface, but the removal rates where significantly enhanced in NaCl, where indirect oxidation by hypochlorite, formed by the electrolysis of chloride, increased the apparent reaction rate constants of the PAHs by a factor of two to six. The oxidation rate of naphthalene was in all experiments showed to follow second order dependence on the naphthalene concentration, whereas the oxidation rate of fluoranthene and pyrene followed the expected first order reaction kinetics with comparable values of the rate constants. Reducing the NaCl electrolyte concentration at constants current density decreased the removal rate of all three PAHs providing evidence for the importance of the indirect oxidation mechanism in the degradation of the PAHs.

The proof-of-concept study was conducted both by a direct treatment approach and an intermixing-with-oxidant approach, where the contaminated water was intermixed in different ratios with an electrochemically generated oxidant solution with a free chlorine concentration of 2 gL-1. Both strategies resulted in a successful degradation of 5 PAHs to fulfil the discharge limit on 0.010 µgL-1. The intermixing-with-oxidant approach can also be applied as a method to address the actual sediment matrix.