RICERCA ITALIANA     

Engineering of advanced electrochemical processes for the treatment of industrial effluents

Università degli Studi di Roma "La Sapienza"

Abstract

In the last years particular attention has been devoted to the different methods to oxidise organic and inorganic substances in order to meet the strict water quality standards. This has caused an increasing development of industrial and municipal wastewater treatments, which requires nowadays the availability of innovative technologies in addition to traditional biological and physico-chemical methods. Electrochemistry offers, unique reaction conditions, as the working electrode has at the same time the features of an easily recyclable heterogeneous catalyst and the capacity of feeding quantitatively and selectively the simplest and most economic reagent: the electron. This allows to operate under extremely mild operating conditions and, moreove to achieve a relatively high efficiency even when small plants are used. In this research direct, indirect, mediated and combined electrolytic processes will be opimised and tested for the removal of pollutants from industrial effluents both organic (tannins, chlorinated saturated and unsaturated hydrocarbons wastewater from plastics industry whith ethylene glycol) and inorganic (wastewater from the manufacturing of phosphorus-based flame retardant and ammonia nitrogen), for whom convenient remediation actions are not already available.
This creates the need for novel methods and processes to be proposed and their engineering design criteria established.
An economic analysis will complete an overall feasibility study aimed at enlightening the most promising low cost process opportunities for the different industrial wastewater considered.
This research final goal is to realise pilot scale prototypes charachterised by a high versatility which makes them suitable for different modality of treatment.
The final part of the Program will consist in performing field tests at the industrial sites. <<<

Principal Investigator

Carlo Merli Università degli Studi di ROMA "La Sapienza"

Research Objectives

The sustainable development of civil and industrial activities involves a suitable energy management and a responsible protection of the environment. Applied electrochemistry is rapidly assuming a key role in the development of environmentally friendly processes and processes for energy conversion and for exploitation of renewable energetic resources. The electrochemical reactor, as widely stressed, offers unique reaction conditions, since electrodes have simultaneously the characteristics of a heterogeneous catalyst, even recyclable, and the capacity to dose, in a quantitative and very selective way, the simplest and cheapest reagent, the electron: thus, unlike the most usual classical methods, extremely soft operation conditions are ensured.
This has made it possible to apply electrocatalytic processes to several fields of industrial activities (chemical, pharmaceutical, food industry, etc.), to the environmental protection (effluent treatment, soil remediation, etc.), to the production, conversion and accumulation of energy (cells and batteries, fuel cells, extra-pure hydrogen production, super condensers).
There exists a vast literature on feasibility studies for destruction of pollutants from industrial wastewater by means of electrochemically generated homogeneous phase mediators however the available studies arrive at most at determining the kinetics of pollutants destruction at ambient temperature, without attempting to optimise the yield of the electrochemical reactor.
There exists a vast literature on feasibility studies for destruction of pollutants from industrial wastewater by means of electrochemically generated homogeneous phase mediators. However the available studies arrive at most at determining the kinetics of pollutants destruction at ambient temperature, without attempting to optimise the yield of the electrochemical reactor.
Therefore the main goal of this project is to design and realise pilot scale prototypes both of an electrochemical generator for liquid phase redox mediators and of the enhanced electro-fenton system for the removal of pollutants from industrial effluents, to be tested in an in-field application. The final goal is to offer a well-assessed novel method of treatment of the wastewater which will allow to decrease the treatment costs for industrial effluents from particularly strategic sectors such as textile and tanning industries and for a those effluents for which convenient remediation actions are not yet available.

1)optimisation of combined processes: to this aim either anodic direct oxidation or indirect oxidation will be tested. In particular the objective will be the enhancement of pollutant transformation by combining direct anodic oxidation with indirect cathodic oxidation treatment (electro-fenton) .
This kind of coupled treatment presents a character of remarkable innovation considering the international state of art of research in this field of interest. The combined oxidation can occur either in a parallel modality, that is in an undivided cell, or in a series modality realised forcing the solution to pass first in a compartment and then in the other compartment of a divided a two chamber-cell The sequential combined treatment, occurring in parallel, if possible, would represent the optimal compromise between costs and reaction times since the two processes occur at the same time, that is exploiting the same current, with clear economic benefits.
Two different wastewaters, one inorganic and the other mainly organic, will be studied. Oxidation efficiency will be optimised by changing electrode materials and operative conditions.


2) Optimisation of advanced electrochemical oxidation processes (AEOPs), especially of an innovative electrochemical method which uses the Electro generated Fenton Reagent (EFR). This method takes advantage of the oxidative characteristics of the hydroxyl radical generated in the reaction between ferrous ions and hydrogen peroxide (Fenton reaction). This species is generated in situ via the electrochemical reduction of oxygen on gas diffusion electrodes. The same process has shown a high efficiency, as compared to other systems, for the removal of various types of organic pollutants, in particular for the treatment of wastewater from vegetal tanning, in combination with an Electro-precipitation process (EP) which uses sacrificial iron anodes. The Electro-precipitation-Electro-coagulation process allows, on the other hand, a high energy saving with respect to the mineralization process, since a quick and (almost) complete removal of organic pollutants occurs with the involvement of a low charge consumption.

3) Application of the electrochemical generator for the hybrid simultaneous production of redox mediators via anodic and cathodic processes, which will be applied for the homogenous phase destruction of pollutants. The processes that can be used to remove pollutants via an oxidative route are based on the anodic generation of Cl-/Cl2 and other oxidants, including radical species, and the cathodic production of H2O2; the simultaneous anodic and cathodic co-generation of mediators and their co-participation in oxidative homogeneous phase reactions, offers possible advantage for the process of elimination of pollutants; till now it has never been an objective of a detailed specific study. In particular, the research will be oriented towards the removal of tannins, dyes, ammonia nitrogen and chlorinated hydrocarbons.

4) Reaction mechanisms studies: the final goal of these studies is to provide a fundamental knowledge of the controlling reaction steps, as well as of defining reliable reaction kinetics expressions, accounting for the major influencing factors

5) Process simulation and feasibility study: simulation models will be developed for the integrated treatment processes (innovative and conventional technologies), capable to predict the performance of these processes and to allow the partners to select, also by a technical-economical feasibility study, the best process solutions for the various types of studied waste waters.

6) The design of the pilot-scale plant and construction of the prototype. These objective will be reached through the followings steps:
° The choice of the flow rates to be treated in a pilot-scale reactor, computation of the volumes of single sections of the generator, computation of the electrode surfaces, choice of the reactor materials (other than the electrodes);
° Optimisation of the placement of the electrodes and their geometry;
° Construction of electrical contacts and electrical parts;
° Definition of the way of connecting and designing all the other apparatuses comprising the pilot-scale set-up (pumps, aerating equipment- if will result necessary-, current feeding equipment, piping, security valves, off-vents, etc.). <<<

Timescale

24 months

National and international background

Several innovative technologies have been recently considered as a possible alternative for treating industrial wastewaters containing pollutants recalcitrant to traditional removal methods. Electrochemical treatments appear to be very promising [1-4], due to the ease in controlling the process of either oxidation or reduction, by “dosing” the electron as the reagent. Further the electrode assumes the role of a heterogeneous catalyst, easily reusable.
Electrochemical treatment is characterised by several advantages: mild operating conditions (room temperature, ambient pressure), easy control of process kinetics (the electric current or the electric potential applied can be easily modulated), no addition of chemical reactants is required, in general. In addition, electrochemical methods offer a relatively high abatement efficiency even in low-scale plant, with a noteworthy elasticity of use. Electrochemical techniques proved efficient in destroying a variety of pollutants as: ammonia [5], nitrites [6], benzoquinone [7], benzene [8], thiourea dioxide [9], phenols [10] , chlorophenols[11], dyes [12], formaldehyde [13], cyanides [14], toluene [15], alcohols [16-18], hydrocarbons [19].
A recent trend is to electrogenerate oxidant agents such as hydrogen peroxide, with the advantage not to leave inorganic residue after reaction with organics. Active free radicals with hard oxidizing properties may be produced by adding ferrous ions according to fenton reaction. [20-30]
The cathodic regeneration of Fe2+ in acidic solution allows providing this reagent continuously to the system, thus preventing the rapid decrease in iron ions, which is observed in chemical Fenton method[31-48]. The main disadvantage related to the limiting diffusion of oxygen from solution to cathode surface and low oxygen solubility is completely overpassed by employing a gas diffusion cathode [49-53]. This kind of electrodic material shows higher current efficiency in the cathodic reduction of oxygen and remarkably high productions of hydrogen peroxide which is explained by the fact that in this electrode the diffusion of the involved species does not represent a limiting step thus minimising the occurrence of side reactions with comparable performances in a wide range of potentials. A further advantage presented by the oxygen diffusion cathode is the possibility of maintaining a high production yield also in the air flow with a obvious notable saving.
The same process has shown a high efficiency, compared to others, for the removal of various types of organic pollutants [54-59], in particular for the treatment of wastewater from vegetal tanning, in combination with an Electro-precipitation process (EP) which uses sacrificial iron anodes [60]. The Electro-precipitation-Electro-coagulation process allows, on the other hand, a high energy saving with respect to the mineralization process, since a quick and (almost) complete removal of the present organic pollutants occurs with the involvement of a low charge consumption.

Also Cl2/Cl- mediated electrooxidation is a particularly challenging tool for biologically recalcitrant contaminants that can be found in many industrial effluents, including tannery wastewater [61-62]. Previous studies have also shown its effectiveness in the treatment of textile wastewater as the chlorides are present in the electrolyte and the process proceeds via formation of 'active chlorine' [63]. However, results suggest that reactor optimization should be achieved for an industrial scale application to be proposed. Indirect electro-oxidation of pollutants in wastewater by electro-generated chlorine has been studied widely, because of the low cost of chloride (that is often already present in industrial effluents, e.g. textile industry) and the relatively high solubility (ca. 0.05 M) and strongly oxidising properties of 'active chlorine'. In this process the anode material is fundamental, as electrochemical oxidation of Cl- ions to Cl2 should proceed at a high rate. Moreover the anode should be inactive towards other species present in the electrolyte, in order to prevent active sites being blocked by adsorption. The results of the study on materials capable to generate Cl2 indicate the coatings composed of Pt + Ir and of Co + Pd, deposited on a Ti base, good electrocatalysts for the chlorine evolution reaction and, as such, potentially suitable for different applications of Cl2 mediated electro-oxidation [64]. The production of this highly oxidative reagent occurs by the surface reaction of electrochemical oxidation of chloride ions at the anode[65].

To the best of our knowledge, there are several examples of integration of electrochemical processes with chemical or biological treatment, while poor information is available as regards the possible combination of only electrochemical processes both of direct or indirect type which appears to be particularly appealing being realised with no chemicals reagent.
The simoultneuos action of efficient anodes and oxidant species electrogenerated in the cathode was expected to increase notably the oxidation rates of organics.
In this frame, several anode materials have been tested, but some of them presented a rapid loss of activity due to surface fouling (glassy carbon [66]), others led to a partial oxidation of pollutants without obtaining their complete incineration (Ti/IrO2 [67-68]) and others showed a limited service life (Ti/SnO2 [69]). On the contrary, synthetic boron-doped diamond thin films, deposited on p-Silicon substrates via the hot filament chemical vapour deposition technique (HF-CVD), with its high anodic stability, wide potential window of water discharge and low background current is undoubtedly a promising material for many electrode applications [70-77].
The combined oxidation can occur either in a parallel modality, that is in an undivided cell, or in a series modality realised forcing the solution to pass first in a compartment and then in the other compartment of a divided a two chamber-cell . In the second case the two processes occur at the same time, that is exploiting the same current, with clear promising economic benefits.
It should be observed, however, that the application of the electrochemical processes to a specific removal problem remains a rather complex task. Optimising the efficiencies related to several compounds that are to be eliminated, together or in successive steps, and defining the characteristics of the cycle that one will use to achieve a maximum of efficiency and a minimum of energy spent is a rather complex research process.

It has to be underlined that the three research groups involved in the project are both qualified and used to working together. They are, in fact, part of the working party TERSI (Electrochemical technologies for the recovery of industrial wastes and effluents) and of AIDIC (Italian chemical engineering society), whose patronage has made it possible so far the organisation of an annual congresses (ERA 2000 at Saline Ioniche; ERA 2001 at the University of Genoa; ERA 2002 at the University of Torino, ERA 2003 e ERA 2004 at the University of Roma, ERA 2005 at the University of Cagliari).) dealing with topics strictly related to the present project. Furthermore, issues of international journals such as Annali di Chimica and Journal of Applied Electrochemistry were devoted to congress contributions. <<<