Environmental Water Treatment Knowledge: The Meaning and Function of Electrocatalytic Oxidation

Because the efficiency of direct oxidation by strong oxidants cannot reliably meet treatment requirements, more effective oxidation technologies had to be developed. In 1987 Gaze and co-workers proposed Advanced Oxidation Processes (AOPs), which solved the problems of conventional oxidation and have attracted growing attention for their distinctive advantages. The most notable feature of advanced oxidation is that hydroxyl radicals (HO·) are generated in the oxidation system by one means or another, and these radicals act as the principal oxidant reacting with organic matter; the reaction can also produce organic radicals or organic peroxy radicals that continue the chain, so that organic matter is fully or partially decomposed. [2]

Advanced oxidation technologies are numerous and varied, and electrocatalytic oxidation is one form of them. Compared with direct oxidation by ozone, the hydroxyl radicals (HO·) generated in an electrocatalytic oxidation system react roughly five orders of magnitude (about 100,000 times) faster, show no selectivity, and can react with virtually all organic compounds. The effect of advanced oxidation is therefore stable and does not vary with changes in the residual organic matter in the water, which is why environmental engineers attach such importance to it. [3]

Several refractory organic compounds are compared below in terms of their reaction rate constants with ozone and with hydroxyl radicals.

How electrocatalytic oxidation differs from electrolysis and electro-Fenton

Difference from electrolysis:

• The oxidizing power of electrolysis is supplied by the anode.

• The current density of electrolysis is not lower than 300 A/m2.

Difference from electro-Fenton:

• In electro-Fenton the anode is iron.

• This technology uses a DSA anode.

Chemical equations of electrocatalytic oxidation

Oxidant: hydrogen peroxide (the highest oxidizing capacity available).

Cathode reaction: H2O2 + e → HO· + OH-

Anode reaction: H2O2 - e → HO· + 1/2 O2 + H+

In an electrocatalytic oxidation system the anode reaction dominates, and the utilisation rate of hydrogen peroxide exceeds 90%.

Features and advantages of electrocatalytic oxidation

Process advantages

1. Many industrial plants now separate clean and dirty streams and treat low-strength and high-strength wastewater separately: the low-strength stream is treated and recycled, while the high-strength and high-salinity streams are treated to discharge standard.

2. Few equipment items, few control points, a simple process and straightforward operation. The entire oxidation stage requires only a single electrocatalytic oxidation reactor.

3. Low engineering investment and low operating cost. Catalytic oxidation reaction time is generally 10 to 30 minutes, and the dosage of 30% hydrogen peroxide is 100 to 300 mg/L (the higher the influent COD, the longer the reaction time and the larger the peroxide dose required). The direct operating cost is CNY 0.15 to 0.45 per tonne of water.

Performance advantages

1. Oxidizing power: 1.36 times that of ozone, comparable to the oxidizing power of the Fenton reaction.

2. Cleanliness: no iron salts are added, no pH adjustment is needed, and no solid particulate catalyst is required.

3. Economy: the utilisation rate of hydrogen peroxide exceeds 90%, and the cost of reducing COD by 50 in the wastewater is only CNY 0.7 per tonne.

4. A unique cathode material: the cathode uses anti-fouling processing and surface materials, giving strong fouling resistance; the anode has a long service life and stable performance.

Advanced treatment of biologically treated effluent

In advanced treatment projects for biologically treated effluent the water quality is relatively good, so a small oxidant dose and a short oxidation time can be used. Typically the oxidant dose is 10 to 20 mg/L and the catalytic oxidation residence time is 10 to 30 minutes.

Pre-biological treatment of high-strength wastewater

1. Stable catalytic efficiency, with an oxidant utilisation rate as high as 95% or more and a long service life for the catalytic electrodes.