The electrocoagulation and electroxidation processes were first applied in the 1960s and went on to become the focus of many developments and patents. Anyway, they never enjoyed acknowledgement and widespread diffusion in the field of wastewater treatment. There were two main reasons for this: the high production of sludge resulting from the corrosion of the electrodes on the anode (generally iron or aluminium) and the high costs of implementing the process.

SA Envitech developed an innovative electro-oxidation process that uses RECAM^® nanostructured electrodes both as cathodes and anodes, with no sludge production.

Figure – Concept of electroxidation cell with RECAM^®

The contaminants electroxidation is encouraged through two distinct approaches:

• Indirect oxidation: a mediator is generated within an electrochemical matter to activate oxidation (Agents generated by the anode, which are responsible for the degradation, may be chlorine, hypochlorite, hydrogen peroxide, ozone or mediator metals)
• Direct anodic oxidation: contaminants are degraded on the anode surface by electrochemical conversion and combustion. During the electrochemical conversion, the organic compounds are only oxidized partially, whereas during the electrochemical combustion they are completely mineralized into carbon dioxide and water.

In presence of RECAM^® electrodes, the main reactions that take place in the electro-oxidation cell are the following:

at the anode:

2H₂O_(l) → 4H⁺_(aq) +O_2(g) + 4e^-

at the cathode:

O_2(aq) + 4e^- → 2H₂O₂ + 4OH^-

Furthermore, when the anode’s potential exceeds 1,51 V ozone is produced:

H₂O + O_2(aq) → O₃ + 2H⁺ + 2e^-

During the process various oxidant agents are therefore produced. These are oxygen, ozone and hydrogen peroxide, as well as free chlorine and free radicals such as ClO^•, Cl^• and OH^• in presence of a significant concentration of chlorides in the liquid subject to treatment.

When applied to wastewater with significant concentrations of chlorides, the following equations can be observed:

at the anode:

2Cl^- → Cl₂ + 2e^-

6HOCl + 3H₂O → 2ClO₃^- + 4Cl^- +12H⁺ +1.5O₂ + 6e^-

at the cathode:

OCl^- + H₂O + 2e^- → Cl^- + 2OH^-

and consequently in the solution:

Cl₂ + H₂O → HOCl + H⁺ + Cl^-
HOCl → H⁺ + OCl^-
organic contaminant + OCl^- → subproducts → CO₂ + Cl^- + H₂O

APPLICATION FOR REMOVAL OF NITROGENOUS COMPOUNDS

The process is also particularly effective for the removal of nitrogenous compounds and ammonia. Under ideal conditions, in presence of a high chloride concentration, the oxidation of the ammonia, that could theoretically lead to the complete conversion into N₂, will occur. The direct and indirect oxidation of the ammonia occurs with the reactions below:

In fact, the chloride concentration does not vary, having the exclusive role of catalyst except for a percentage decrease due to the percentage of Cl₂ that turns into gas and does not return to the solution.

PROPERTIES OF RECAM^® ELECTRODES

When RECAM^® is subjected to mechanical compression it constitutes an electrode with the following properties:

• increased electrical conductivity, thanks to the unique structure of graphene cells
• behavior of the catalyst: no process of corrosion of the electrode effects the anode
• does not lead to the formation of sludge and sediments in the test cell
• promotes the formation of reaction subproducts with a high oxidant potential
• acts as a catalyst
• contains pH variations.

Therefore the electrodes’ characteristics are the company’s added value. This process, developed on a large scale, offers obvious economic advantages thanks to the lack of sludge production.

In the table below are reported the main parameters that affect the electro-oxidation process with RECAM^®

Table – Main parameters that affect the electroxidation process with RECAM^®.

Figure – Electroxidation unit with RECAM^®.