Electrochemical degradation

Electrochemical degradation uses electric current and electrodes posited in a surfactant containing solution to oxidise them into carbon dioxide. The positive electrode (anode) oxidises compounds in solution by removing electrons. Electrons flow to the negative electrode (cathode) that reduces compounds by donating electrons. Organic compounds can be oxidised indirectly using chorine and hypochlorite generated by the anode at high chloride concentration. Another indirect oxidation uses hydrogen peroxide. Direct anodic oxidation generates physically adsorbed “active oxygen” (adsorbed hydroxyl radicals •OH) or chemisorbed “active oxygen” (oxygen in the oxide lattice, MOx+1).¹⁷⁵ Anode materials can include, for example, dimensionally stable anodes (DSA), such as RuO2 or ZrO2 coated Ti, thin film oxide anodes (PbO2, SnO2), noble metals (platinum) and carbon-based anodes. New synthetic boron-doped diamond (BDD) thin film elec-trodes have received attention due to their particularly high efficiency to degrade organ-ics. Lissens et al. ¹⁷⁶ studied electrochemical oxidation anionic (sodium dodecylben-zenesulfonate) and cationic (hexadecyltrimethyl ammonium chloride) at a BDD (boron-doped diamond) electrode. Degradation was monitored by measuring TOC of the solu-tion. They reached 82 % removal of surfactants and stated that alkaline pH enhanced the oxidation process.

Louhichi et al.¹⁷⁷ used the electrochemical oxidation on BDD-electrode of wastes wa-ters containing surfactant sodium dodecylbenzenesulfonate (SDBS) and concluded that NaCl seemed to be most efficient electrolyte in surfactant oxidation monitored by measuring COD of the solution. Ciorbaet al.¹⁷⁸investigated an electro-coagulation pro-cess of anionic, cationic and non-ionic surfactants with an aluminium electrode and got a 40 to 60% COD removal.

SUMMARY

The character of surfactants; their effect on foaming, toxicity and biodegradation, is highly dependent on the surfactants structure (alkyl chain length and branching). Sodi-um dodecyl sulfate (SDS), an anionic surface active agent, is a common detergent in hygiene care and cleaning products due to its easy hydrolysis and rather environmental friendly nature, when compared to other surfactants. Its vide use has made it a popular research target and, is also used in experimental part of this thesis.

A wide variety of different surfactant determination methods are available and, again the surfactant structure determines the best technique for the analysation. Careful sam-ple preparation can ease significantly the final separation and detection, even though the prepreparation methods usually consume more time than the actual instrumental analy-sis. Chromatographic methods are used for sample separation from impurities or other surfactants. Liquid chromatography, and its modifications, being the most used methods for both qualitative and quantitative analysation of environmental and wastewater sam-ples.

Detector choice depends on the nature of the target compound, the sample matrix and the need for qualitative or quantitative determination. Mass spectrometry (MS) can pro-vide exact results with high accuracy and ability to distinguish different homologues of surfactants, but is expensive and thus not suitable for routine analysis. Other detection methods, such as conductivity detection and evaporative light scattering, are not as ac-curate as MS, but still provide good separation efficiencies and can also be used for rou-tine control.

Foaming can be a problem in a paper- and board mills and a wastewater treatment plants (WWTPs). Foam managing is mainly handled using antifoamers of physical re-moval methods. The presence of surfactants in environmental- and wastewaters increas-es the carbon load of the water. Surfactants need to be removed from the wastewater before the release back to the environment. A wide range of different methods using chemical-, physical- and biological techniques have been developed and provide an ef-ficient removal rates for lager variety of surfactant classes.

EXPERIMENTAL PART

7 OBJECTIVES

The experimental part of this thesis is composed of three themed parts where anionic surfactant, sodium dodecyl sulfate (SDS), plays the leading role. The first part deals with the determination of SDS by high-performance reversed-phase liquid chromatog-raphy (HPLC-RP), combined with electrical conductivity detection (ECD)(Chapters 8.2, 10.2 and 11.1). The second part focuses on the development of accelerated aeration test (Chapters 8.3, 10.3 and 11.2). The third part deals with the removal of SDS by using a flocculation method (Chapter 8.4, 10.4 and 11.3).

Solvent extraction spectrophotometry108,109,110

(here abbreviated SES) using cationic dye (e.g. ethyl violet) as a colouring agent, is one of the most used determination methods of anionic surfactants (Chapter 8.1 and 10.1). It is a simple, rather sensitive and cheap method for analysing the total amount of SDS in a sample. However, it also consumes large volumes of toxic organic solvents and sample matrix can easily interfere the re-sults by alternating the volume of the colouring agent that transfers into the organic phase. Thus, another common but more sophisticated anionic surfactant detection meth-od, high-performance reversed-phase liquid chromatography (HPLC-RP), was tested in SDS determination.^73,75,79

SDS is known to hydrolyse rather easily over the time, and the process can be accelerat-ed by heating and pH change.^29,30 Thus, the long-term storage of SDS solutions is doubtful. One week time monitoring test was performed to get information about the shelf life of SDS solution. SDS concentration was determined with both SES- and RP-ECD-methods. The accelerated hydrolysis of SDS would be an alternative removal method of SDS from wastewaters. SDS hydrolysis experiments by heating and pH change was performed to define optimal hydrolysis conditions.

The effects of additives (salts and retention aids) to SDS content of white water were examined with RP-ECD and SES-method and the results were compared to see if there are any significant differences. It was assumed that the SES does not distinguish intact

SDS from hydrolysed parts (free sulfate head), and positive interference may occur.

White waters also contain large amounts of fibres and additives (salts and retention aids) that might disturb the results. RP-ECD was assumed to be able to distinguish hy-drolysed parts of SDS from the intact molecule and determine the free SDS content of the samples. It is rather important to know the amount of free SDS (SDS in monomeric form) in the sample since only the free SDS can affect the surface tension and foam generation of a liquid.

In addition, during the SDS analysis by RP-ECD some problems occurred in sample purification and syringe filtration. Hence, tests with different filter membrane materials (GHP, nylon) and solid-phase extraction (SPE) method, were performed, and proce-dures and results are also included in this work.

SDS was not the only surfactant used as a foaming agent in foam forming. Miranol Ul-tra is an amphoteric surfactant and cannot be determined with solvent exUl-traction spec-trophotometry method. However, due to its imidazole based structure it can be detected with UV-detector (205 nm). Thus, RP-UV tests were carried out, including calibration curves and effect of salts and retention aids.

The second part of the experimental work focused on the development of laboratory scale measurement system for the analysis of foaming tendency of SDS containing wastewater during aeration. The aim was to imitate real aeration tanks of wastewater treatment systems in paper-, and board factories and observe how SDS addition affects the foaming behaviour of the samples in the WWTP aeration tanks.

Two criteria in the selection of air flow rate for the aeration tests was applied. Firstly, the air flow values at wastewater treatment plants (WWTP) were considered and sec-ondly, the target was to develop an accelerated test (test period max 2-3 hours). Since the air flow rate in the real aeration tank is 0.5-1.5 m³/h per 1m³, it was calculated that the air flow rate should be at least 0.25 l/min in the laboratory vessel (water volume 10 l). According to preliminary laboratory tests, air flow rate 0.6 l/min, which corresponds to air flow rate 3.6 m³/h at WWTP, was high enough to generate foam when the SDS concentration of white water sample was 50 ppm. Thus, 0.6 l/min was chosen for the

further experiments. When analysing water samples having low SDS concentration (<

50 ppm) air flow rate 1 l/min could be more useful.

The aeration procedure was generated based on the preliminary tests. Aeration time 1.5 h per sample and foam dying time 15 min were found to be suitable for the foaming behaviour observations. The purpose of dying foam observations was to study how the other agents of the sample (fibres, additives) affect the foam stability and behaviour.

Flocculation tests were the third part of the experimental work. The aim of the floccula-tion tests was the examinafloccula-tion of precipitafloccula-tion of SDS from pure- and white water sam-ples using trivalent cations Al³⁺ and Fe³⁺ as coagulants. The objectives were to study the effects of coagulant dosage and pH on the precipitation efficiency of SDS. The determi-nation of SDS was done by using SES-method. Precipitation/flocculation is a common wastewater treatment method, and the purpose of these experiments was to define opti-mal conditions for SDS removal from wastewaters.^3,18,151

8 DEVICES