Electrochemical Technologies for Removing Petroleum Hydrocarbons from Produced Water
The existence of indirect or mediated oxidation with different heterogeneous species formed from water discharge has allowed the proposal of two main approaches for pollution abatement in wastewaters by EO:20
• the electrochemical conversion method, in which refractory organics are selectively transformed into biodegradable compounds – usually carboxylic acids – with chemisorbed ‘active oxygen’; and
•
the electrochemical combustion (or electrochemical incineration) method, in which organics are completely mineralised – i.e.,
oxidised to CO2 and inorganic ions – with physisorbed *OH; this radical is the second strongest oxidant known after fluorine, with a high standard potential (Eº = 2.80 V vs. SHE) that ensures its fast reaction with most organics giving dehydrogenated or hydroxylated derivatives up to conversion into CO2.
In both cases, significant cell voltages are applied to the electrochemical cell for the simultaneous oxidation of pollutants and water, thus maintaining the anode activity. The use of low cell voltages
avoiding O2 evolution frequently causes the loss of anode activity, because some by-products formed from direct anodic oxidation can be adsorbed on its surface. The nature of the anode material influences strongly both the selectivity and the efficiency of EO.
Figure 3 shows some electrochemical plants and cells designed for this technique and operating in batch mode to obtain the maximum solution decontamination. A good design allows to optimise the mass transport coefficient for reaching the maximum current efficiency.
The most important experimental parameters measured in EO are the abatement of COD and/or total organic carbon (TOC) measured by the percentage of degradation or mineralisation achieved. The latter data are also used to obtain the simple current and energetic efficiency parameters that characterise the EO process.
It is important to note that high dissolved concentrations of NaCl are found in produced waters. Therefore, EO via OH radicals is not the only oxidation mechanism that occurs when electrochemical
0 5 1 4 6 7
Figure 4: Organic Pollutants Removal from Produced Water by Controlled Current Density Electrolysis (89 mA/cm-2)
2 AB 2 3 4 C 3 1 5 6 7
5
101520253035 Retention time
Initial sample (black line) and after electro-oxidation (red line). Experimental conditions: T = 25 °C; electrode area = 19 cm2 flow rate = (A) 0.25 dm-3
h.
Produced water samples contained approximately 20–30 mg dm/3 of benzene, toluene, ethyl benzene and xylenes and 5 mg dm/3 of phenol.
Key: (1) phenol; (2) benzene; (3) toluene; (4) ethyl benzene; (5) o-xylene; (6) m-xylene; (7) p-xylene. Inset: (A) electrochemical reactor: (1) electrolyte inlet; (2) cathodic electric contacts; (3) electrolyte outlet; (4) stainless steel AISI 304 bars as cathodes; (5) flow direction; (6) anode and (7) electric anode contact.
(B) MeV image of Ti/RuO2TiO2SnO2 anode. (C) Image of mesh anode.
, NaCl = 15,000 g dm/3; pH = 6.86;
chlorine and/or chlorine–oxygen species that can oxidise organic pollutants in the bulk until overall mineralisation;18
and •
EF process, an emerging technique in which organics are mineralised with homogeneous *OH formed from Fenton’s reaction
between added catalytic Fe2+ and H2O2 electrogenerated from O2 reduction at a suitable cathode.17
In both cases, petroleum pollutants are competitively destroyed by direct anodic oxidation and by reaction with heterogeneous *OH and other reactive oxygen species, and weaker oxidants produced from anodic oxidation of water and anions of the wastewater.17,18,20,21
In IEO with
Combined electrochemical technologies could be used to reduce the effects of environmental disasters.
technologies are used; other oxidants can be also generated on the anode surface and will consequently oxidise organic matter in an IEO process.
Indirect Electro-Oxidation (IEO) with Strong Oxidants In IEO (see Figure 2), two approaches are mainly used:
• IEO process with active chlorine, in which direct anodic oxidation of chloride ion present in the effluent leads to the formation of free
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 2
active chlorine, the type of anode material chosen merits special attention, because it determines the predominant oxidants produced during the electrolysis of chloride solutions. Electrochemical systems similar to those used for EO (see Figure 3) are used in the IEO treatment of petrochemical hydrocarbons effluents with active chlorine.
Applications of Electrochemical Methods
A limited number of reports have been published concerning the application of EO and IEO for removing petroleum hydrocarbons from petrochemical wastewaters.7,9,11,13–17
Table 1 summarises the most
relevant results obtained for the degradation of petroleum hydrocarbons by direct EO and IEO and EF processes.
A closer look at Table 1 corroborates the high mineralisation obtained through EO with different anodes. For example, Santos et al. investigated the electrochemical remediation of oil extraction industry wastewater using Ti/Ru0.34Ti0.66O2 anode.9
The authors obtained
a COD reduction of 57 % in an oily sample treated for 70 hours at 50 °C with a current density of 100 mA/cm2; the slow rate of COD reduction could be attributed to the occurrence of secondary
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