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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.46 n.4 Concepción dic. 2001

http://dx.doi.org/10.4067/S0366-16442001000400002 

"OXIDATION-REDUCTION POTENTIAL (ORP) IN PREPARED AND
INDUSTRIALLY TREATED WATERS"

RAFAEL MARÍN GALVÍN (*), JOSÉ MIGUEL RODRÍGUEZ MELLADO,
MERCEDES RUIZ MONTOYA AND CARMEN JIMÉNEZ GAMERO

Departamento de Química Física y Termodinámica Aplicada
Facultad de Ciencias, Universidad de Córdoba
E-14004-Córdoba (Spain)
Phone: 34-57-218617/18/19; FAX:34-57-218606
E-mail:QF1ROMEJ@uco.es
(Received: March 2; 2000-Accepted: September 2; 2000)

ABSTRACT

A linear relationship between oxygen contents (as log %O2) and EH values for synthetically prepared waters has been obtained. Highly oxygenated waters showed values around EH=-0.040V and rH=13.0, while poorly oxygenated waters showed values around EH=-0.145V and rH=12.2. Such values were independent of the salinity. The presence of two active redox pairs typically encountered in natural waters such as "Fe3+/Fe2+" and "Mnn+/Mn2+" also contributed to the EH and rH values by increasing the ranges of variation with respect to those obtained in their absence.

In industrial drinking water production, dosage of O3 or Cl2 to raw water could serve to sterilise this water by increasing the EH and rH values in 0.210V and 21.8 units, respectively.

In industrial biological treatment of wastewaters, very important reductions in the COD and NH3 concentrations were associated to differences (0.300-0.400V in all cases from one water to another) in EH in aerated water with respect to the EH in the influent non-treated wastewater.

KEYWORDS: Drinking water. Redox potential. Synthetic waters. Wastewaters. Treated waters.

RESUMEN

Se ha obtenido una relación semilogarítmica entre la oxigenación del agua (como %O2) y los valores de EH para aguas preparadas sintéticamente. Las aguas con elevada oxigenación mostraron valores en torno a EH=-0.04 V y rH=13.0, mientras que para aquellas poco oxigenadas los valores fueron de EH=-0.145 y rH=12.2; estos valores fueron independientes de la salinidad. La presencia de dos sustancias redox típicas del agua como "Fe3+/Fe2+" y "Mnn+/Mn2+" contribuyeron también a los valores de EH y rH aumentando los rangos de variación respecto de los obtenidos en su ausencia.

En la producción industrial de agua potable, la dosificación de O3 o Cl2 al agua bruta puede servir para esterilizarla aumentando los valores de EH y rH en 0.210 V y 21.8 unidades, respectivamente.

En el tratamiento biológico industrial de aguas residuales, se asocia la reducción muy importante en COD y concentración de NH3 a diferencias entre los EH del agua residual aireada y del agua residual no tratada. Estas diferencias fueron en todos los casos de 0.300 a 0.400 V en el valor de EH de un agua a otra.

Palabras Clave: Agua potable. Potencial redox. Agua sintética. Aguas residuales. Agua tratada.

INTRODUCTION

Natural waters involve a great variety of redox pairs, being the H2/H+ the most relevant for the ORP (oxidation-reduction potential). The redox potential of the reaction

½ H2 = H+ + e-

can be measured by using a normal H2 electrode and an inert electrode (for example a platinum electrode). Taking into account the expressions of cell potential, the dissociation equilibrium of H2 and the Nernst equation, it can be concluded that [1,2]:

EH = 0.06log[H+] - 0.30logPH2

and considering with Clark [3] that "log(PH2)=-rH", it can be established that [1, 2, 4]:

rH = EH/0.30+2pH

This equation relates the two parameters commonly used to express the ORP in water, "rH" and "EH". High values of rH (>15) and EH of positive sign (or very low EH values irrespective its sign) characterize oxic conditions, and conversely, low values of rH and EH of clearly negative sign are characteristic of anoxic waters [1, 2, 4].

The specific redox state of a given water (natural, wastewater or along its treatment processes) drives its behaviour and the concentrations of different compounds [4-14]. Two circumstances can be found for a water: high oxygenation (high ORP values) and abundance of oxidised species (i.e. Fe3+ and Mn4+ slightly soluble, inorganic carbonaceous compounds, sulphates, nitrates...) and deficient oxygenation (low ORP values) and abundance of reduced species (i.e., Fe2+ and Mn2+ sparingly soluble, organic carbonaceous compounds, methane, sulphides, nitrites...). Such situations succeed one to another depending on the stational changes of the environment for natural waters, or the stage of manipulation for treated, drinking and waste water. In addition, the lower or higher ORP strongly influence the subsequent industrial treatment processes of these waters.

The aim of this paper was to contribute to the study of the ORP in waters in two ways: first, the relation between oxygenation, ORP and concentrations of Fe and Mn for prepared synthetic waters has been studied. Second, two practical applications of the ORP evolution of water along the treatment processes have been made, with the aim to optimize the industrial treatment of water: (a) raw natural water intended for water drinking production and (b) wastewater intended for biological treatment.

MATERIALS AND METHODS

Measurements of the potential vs. time curves were carried out by using a METEX M-6450 CR digital multimeter coupled with a MULTITECH ACER 710 computer. The working electrode was a Metrohm 6.0302.100 Pt electrode and a saturated calomel electrode Metrohm AG 9100 was used as reference electrode. All potentials were referred to the normal hydrogen electrode by subtraction of the potential of the saturated calomel electrode. The electrolytic cell used was thermostated and equipped with a pipe to bubble nitrogen or air into water. Oxygen was measured with a CRISON OXI92 oxymeter with temperature compensation and equipped with a CRISON E090 oxygen electrode.

The analysis of waters was made according to the "Standard Methods" [12]. Measurements of UV absorbance were recorded by using a computerised UV-Visible Perkin-Elmer Lambda 3b Spectrophotometer, after samples centrifugation at 5000 rpm during 5 min, to clarify the liquid [4]. Finally, the total number of coliforms present in the waters was obtained by the usual membrane filter procedure [12]. Synthetic waters were prepared according to the "theoretical composition" method as described elsewhere [1]. Thus, three kinds of synthetic waters were obtained: low salinity, middle salinity and high salinity waters (see table-1). Concentrations of Fe2+, Fe3+ and Mn2+ were obtained by dissolving FeCl2∙4H2O, FeCl3∙6H2O and MnCl2∙4H2O, respectively.

Table I. Preparation of synthetic waters*

Waters under treatment to drinking water production were taken during October-November 1994 in the Villa Azul Drinking Water Plant which supplies to Córdoba City (Spain). This plant produces 150.000 m3 drinking water/day (maximum), the treatment consisting during the study in aeration of the raw water, ozonization, prechlorination with Cl2, settling, filtration through rapid sand filters and postchlorination (Cl2) [13, 14]. Forty-eight samples were taken, distributed in the groups "raw water", "oxidized water" and "filtered water". Water samples without treatment influent to plant were named "raw waters"; "oxidized waters" corresponding to raw waters aerated, ozonized (1-1.5 g/m3 or ozone) and prechlorinated (4.5 to 5.2 g/m3); "filtered waters" were the above ones settled with alum (dosed between 28 to 30 g/m3) and filtered through siliceous sand rapid filters.

Wastewaters under biological treatment were taken during September-October-November 1994 in La Golondrina Wastewater Treatment Plant which treat the total (domestic and industrial) waste waters of Córdoba City. The plant must treat up to 108.000 m3/day and the treatment employed was the following: double screening, removal of sands and greases-oils, primary settling, aeration, secondary settling and sludge treatment (incineration or chemical treatment with Ca(OH)2) [15]. Fifty-seven samples were studied distributed in the series "waste water", "settled water" and "treated water". Waste water samples without treatment influent to plant were named "waste waters"; "settled waters" corresponded to screened waste waters, without sands and greases and primarily settled; "treated waters" finally, were the above ones aerated (to a residual oxygen concentration of 1.5 g/m3) and finally settled after biological treatment.

Two methods were used to obtain waters with a given oxygen concentration:

(a) The sample is saturated with oxygen by bubbling O2 (or, alternatively, air) during 8-10 min. The oxygen is then slowly removed from the water by bubbling N2.

(b) All the O2 is removed from the water by bubbling N2 and the oxygen concentration is slowly increased by bubbling O2 or air.

In both cases the oxygen content is monitored with the oxymeter and when the desired concentration is reached, the water is used.

RESULTS AND DISCUSSION

The rate of electron transfer towards a platinum electrode is associated to the surface characteristics of the Pt. After each measurement adsorbed substances cover the electrode preventing new measurements [12]. For this reason, the electrode exhibits a "memory effect" which strongly affects the measurements of the following samples. Several cleaning methods were used to overcome these problems [1, 9, 16-18]. Among these methods, and after a variety of essays, we have selected the following cleaning procedure before each measurement: introduction of the Pt electrode during one minute into sulfo-chromic mixture (6 g of K2Cr2O7+100 ml of concentrated H2SO4 in 250 ml of total dissolution) and rinsing with abundant distilled water. So, the potential vs time curves and the EH and rH values obtained were highly reproducible.

(a)Synthetically prepared waters

To study the influence of oxygen on waters ORP, different oxygen concentrations were taken for prepared waters according to method (a) described in the precedent section. Once the electrode is immersed into the water, the recording of ORP starts. This recording was made in a continuous mode, maintaining the electrode into the water until the end of the experiment. The ORP varied with the measurement time in the general shape shown in Figure 1, that is, reaching a limiting value at long times. The establishment of the end value of ORP is driven by the diffusion of charged species towards or from the electrode and by the adsorption/desorption of such species on the electrode surface, explaining (at least qualitatively) the shape of the EH vs. time curves. This general behaviour was found for all synthetically prepared waters, irrespective their composition. Thus, EH is the "equilibrium oxidation-reduction potential" and corresponded to ORP when the equilibrium in the interface is reached. It must be remarked that the measurement corresponded in all cases to a unique oxygen content and the stabilisation time was reached when the EH values changed less than 5 mV (being this quantity the accuracy in EH), in this case after 1300 seconds. Table-2 presents the data of %O2, EH, rH and pH for all the prepared waters investigated. As can be seen, higher oxygenations implied higher EH and rH values. Conversely as occurs for a natural aquatic medium, the lower oxygenation implied the higher pH-values. This fact can be explained by taking into account the method used to obtain the actual oxygen concentration: the O2 removing by bubbling N2 (or vice-versa) implies a parallel decreased of CO2 in the medium (this gas is also removed) and, consequently, the increase of pH.


Fig. 1. Potential vs time curves from middle salinity prepared waters. %O2 saturation from down to up: 8, 19, 31, 47, 61, 75 and 86.

Tabla II. Results obtained for synthetics waters*

In prepared waters only the O2 influences significantly both EH and rH values as can be inferred from the linear correlation between EH and log %O2 shown in fig.2. Moreover, synthetic waters with >70% in O2 saturation showed rH values around 13 in accord with the experimental values checked for unpolluted natural waters [1, 2, 9, 19].


Fig. 2. Prepared waters: relation EH vs log %O2 saturation. White circles: high salinity waters; black circles: middle salinity waters; triangles: low salinity waters.

The potential vs. time curves were also studied for prepared waters after addition of Fe2+, Fe3+ and Mn2+ ions at the following concentrations (in mg/l): 0.1, 0.3, 0.6 and 1.0. Waters with Fe3+ were firstly saturated in O2 and then slowly deoxygenated by bubbling N2 (method a), while waters with Fe2+ or Mn2+ were firstly saturated in N2 and then oxygenated by bubbling O2 or air (method b).

Addition of Fe2+, Fe3+ and Mn2+ caused variations in the potential vs time curves with respect to the shapes of the curves obtained in the absence of these ions. The magnitude of the distortions followed the sequence "Mn2+>Fe2+>Fe3+". Likewise, the higher the concentration of the ions, the higher the distortion. This can be explained by taking into account the high adsorption of oxihydroxides of Mn and Fe generated during the experiments that harmed the diffusion processes associated to the kinetics of the measurements of ORP. Moreover, the number of possible oxihydroxides generated for Mn is higher than those expected for Fe [20-22], explaining the above sequence.

On the other hand, the more oxygenated samples showed always the highest rH values and the more positive (or less negative) EH values, with independence of salinity and concentrations of ions (table-2). This could be associated to the reversibility of the process as occurs in nature.

The range of variation of EH values in the samples containing Fe or Mn ranged more widely than in the rest of the samples. This must be related to the occurrence of the new redox pairs "Fe3+/Fe2+" or "Mnn+/Mn2+" in addition to the redox pair oxygen-water, which undoubtedly contributed to the waters ORP value.

(b)Waters under treatment to drinking water production

Potential vs. time curves corresponding to these waters were different from those corresponding to prepared waters in the absence of Fe or Mn. This can be explained by the presence of organic matter that show a strong tendency to adsorption on the Pt electrode surface. In this case the equilibration time for ORP, O2 and pH measurements was 1800 seconds.

Table-3 shows the evolution of the water characteristics along the treatment for drinking water production. Important reductions were observed in Fe, Mn, coliforms, UV-254 nm absorbance and COD-Mn (organic matter to KMnO4) from raw untreated water to final drinking water.

Table III. Characteristics of water under treatment for drinking water production (maximum and minimum values)

The evolution of EH and rH in waters under treatment follows the sequence: "oxidized">"filtered">"raw waters" (see fig.3). The oxidized water presented the higher values due to the dosage of some oxidants (O3 and Cl2) specially in the oxidation-prechlorination step. The residual oxidant was lost by both oxidation and atmospheric diffusion, implying the decrease of the above values in filtered waters with respect to oxidized waters. The raw waters showed the lowest EH and rH values of each group due to the oxidant power of O3 and Cl2, much higher than the air oxidant power. However, the raw waters showed always ORP values corresponding to oxic waters [1, 9, 19].


Fig. 3. Waters under treatment for drinking water production: variation of ORP along the treatment (average values). (A) EH; (B) rH.

Oxidation processes used in water treatment originate the removal of oxidizable species (dissolved iron and manganese, organic matter, etc..). The efficiency of this process must be associated to both the specific dose of oxidant employed and the increase of water ORP. In this study, the dosages used for both O3 and Cl2 were relatively low. Moreover, the raw waters untreated showed a reasonably good oxidation state and low levels of Fe, Mn and organic matters. For this reason, it was difficult to prove an experimental relation between high removing of substances easily oxidized, present in water, and high increases of EH and rH in oxidized waters against raw waters. According to these results, the ORP value is not an adequate parameter for controlling the treatment process in production of water intended for human consumption: it is more reasonable to work by optimizing the oxidant dosages as it is commonly made.

Disinfection of water is made with the same reactants as oxidation, the oxidant playing two roles: chemical action with damage of cellular wall and interference on specific enzymatic reactions for the metabolism of microorganisms [23]. Previous papers [23, 24] showed that the increase of EH in waters associated to the dosification of Cl2 and/or O3 was related to the effectiveness in removing escherichia coli and other microorganisms. Increases of 0.5-0.6V were sufficient to promote the disinfection in less than 30 minutes. In the present study it has been observed the complete elimination of the total coliforms number present in raw waters for increases of EH and rH values in oxidized water against raw water equal to 0.21V and 21.8 units, respectively, lower than those reported in the literature. This can be due to the low microbiological amount in our waters against the higher used in other studies. Moreover, such previous studies considered only laboratory experiments and not practical industrial cases as that studied here.

(c)Waste waters under biological treatment

As occurred for the above mentioned cases, potential vs. time curves presented distortions which can be probably due to the very important amounts of substances present in waste waters, specially organics, which are strongly adsorbed on the platinum electrode surface. Nevertheless, approximately after 1800 seconds the equilibrium was established.

Table-4 shows the evolution of waste water characteristics along the biological treatment: high amounts of COD, BOD5 and NH3 were removed from untreated waste water. Particularly, the EH and rH values followed the sequence: "treated water">"waste water">"settled water" (see fig.4). This can be explained by taking into account that the treated water experienced a process of aeration prior to the biological treatment being relatively well oxygenated and thus relatively oxic. Settled water did not experienced aeration, its very low O2 level was consumed and, in consequence, its reduction state was increased with respect to the wastewater.

Finally, waste water was originally anoxic as is typical in these kind of waters. Nevertheless, the EH and rH values measured in this study were lower than those found in the literature [1, 2, 4, 9] for recently produced waste waters. This can be due probably to the consumption of O2 in these waters along the sewage network pipes that carry waste water to treatment plant.


Fig. 4. Waste waters under biological treatment: variation of ORP along the biological treatment (average values). (A) EH; (B) rH.

Fig.5 shows a non linear relationship EH vs. %O2 for these wastewaters. For waters very poorly oxygenated, (%O2<10%) a wide variation in EH values (-500 mV to -100 mV) can be observed. This has not an evident explanation but it can be assumed that under these conditions other redox species different from O2 can influence the redox state. Among such species, the equilibrium "sulphate-sulphide" could be very outstanding in this case, as well as all the reduction processes used by the waste waters anaerobic microorganisms [4, 8, 25, 26]. For medium-high O2 values (%O2>40%), a linear relationship between EH and %O2 was achieved, indicating that anaerobic processes do not influence the redox state of water.


Fig. 5. Waste waters under biological treatment: relation between EH and %O2 saturation for all the available data.

Domestic waste waters contain high amounts of residual organic substances and are poor in dissolved O2. Consequently, a relationship between organic load and both O2 and reductive state in wastewaters can be expected. Fig.6 confirms that waters with high BOD5 values showed low rH and negative EH values.


Fig. 6. Waste waters under biological treatment: relations BOD5 vs rH.

Biological treatment of domestic waste waters is based on the controlled growth of an aerobic microorganisms culture. This is carried out through water aeration before the secondary settling, in our case up to a residual O2 concentration of 1.5 g/m3. In this way, the highest percentages in organic substances removed from water must be associated to an optimal oxygenation process, and then to a sufficiently low negative EH value or to a sufficient increase in the rH value of aerated water against not aerated water. Thus, decreases in COD and NH3 of water around respectively 450 mg/l and 8-10 mg/l (in absolute values) were statistically associated to increases of 0.300-0.400 mV of the EH value in aerated water against influent waste water.

These variations can be obtained by modifying the air dosage to water, which should promote modifications of the water ORP, better than by operating the plant to obtain a pre-fixed residual O2 concentration. This is used in many industrial plants of biological treatment, especially in anaerobic treatment and to reduce nutrients in waste waters, with high efficiency [27, 28] and could imply important savings in the exploitation of the plant.

CONCLUSIONS

(a) Synthetic waters

- A linear relationship between the logarithm of oxygenation and ORP was obtained, with independence of water salinity: EH ranged between -0.040V and -0.145V and rH between 13.0 and 12.2 units for very oxygenated and poorly oxygenated waters, respectively.

- Addition of Fe3+, Fe2+ or Mn2+ to prepared waters implied variations of the ORP due to the contribution of the redox pairs Fe3+/Fe2+ or Mnn+/Mn2+. Thus, EH and rH ranged, respectively, from -0.020V to -0.150V and from 15.0 to 12.0 units for very oxygenated and poorly oxygenated waters, respectively.

(b) Raw waters and along treatment to drinking water production

- Relations between ORP and oxygenation have been stated for both untreated raw waters and waters under treatment for drinking water production. In all cases, the higher the oxygenation, the higher the ORP measured.

- For waters under treatment for drinking water production the ORP was strongly influenced by the oxidant used, O3 or Cl2. Since such oxidants are much more oxidant than air, the measured ORP increased.

- Disinfection of raw waters was observed with associated increases in the EH and rH values lower than those reported in the literature. Increases of 0.210V (EH) and 21.8 units (rH) in oxidized waters compared to raw waters guaranteed the complete elimination of total coliforms. Nevertheless, the practice indicates that industrial disinfection of water must be carried out always by optimising the oxidant dosages and not operating with any pre-fixed ORP value.

(c) Untreated and along biological industrial treatment waste waters

- A non-linear relationship between EH and percentage of dissolved oxygen has been found for both untreated and under biological industrial treatment waste waters. Low oxygen contents (less than 10-15%) implied a wide variation in EH values, whereas waters with 40% O2 or more presented a small EH change with oxygenation.

- A correspondence between organic load (expressed as BOD5) and ORP has been proved. The more negative EH values or the lower rH values were statistically associated to the higher BOD5 values in the range of 13-470 mg/l of BOD5.

- Finally, important reductions of COD (-85%) and NH3 (-28%) concentrations were associated to increases of 0.300-0.400V in the EH value of aerated water against influent wastewater. This can indicate that the biological industrial process could be optimized better by adjusting the ORP values measured as a function of the yield instead of by the conventional method to obtain residual pre-fixed O2 levels.

ACKNOWLEDGEMENTS.

Authors wish to express their gratitude to the Empresa Municipal de Aguas de Córdoba S.A. for supplying the industrial water samples; to Junta de Andalucía for partial financial support and to the Spanish Ministerio de Educación y Ciencia for a postdoctoral Grant (Grant IN92-B30528630).

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(*) To whom all correspondence should be addressed.