The Influence of the Inorganic Species on Oxidative Degradation of 4-chlorophenol by Photo-Fenton Type Process

An experimental study on 4-chlorophenol (4-CP) degradation in aqueous solutions by advanced oxidation process photo-Fenton type is presented. The efficiency of the oxidation process is determined by the very high oxidative potential of the OH. radicals generated by catalytic and photo-catalytic processes. The presence of the inorganic species inside the reaction medium influences the rate of the oxidation process as function of their nature and concentration. The inorganic anionic species reduce drastically the 4-CP oxidation efficiency by Fe2+/3+ complexing processes, HO. radicals scavenging effect or iron precipitate forming. The decrease of the 4-CP oxidation degree is correlated with the nature of the anions as following: Cl> PO4 3> SO4 2>> NO3 -. The presence of the insoluble inorganic species (bentonite) modifies the oxidation efficiency by additional 4-CP and UV sorption processes, especially at high solution turbidity values.

The presence of chlorophenols in water induces a high pollutant potential due to their toxicity to aquatic life and carcinogenic and mutagenic effects to humans [1].Based on their high efficiency, the AOPs (Advanced Oxidation Processes) can be successfully used in wastewater treatment to degrade the persistent organic pollutants, resistant to biological and classical physico-chemical processes [2][3][4][5][6][7].
The oxidation process is determined by the very high oxidative potential of the HO .radicals generated into reaction medium by different mechanisms.In the case of the AOPs Fenton-type procedure (hydrogen peroxide and Fe 2+ as catalyst), the generation of hydroxyl radicals takes place through a catalytic mechanism in which the iron ions play a very important role [4,[8][9][10][11][12][13], the main reactions involved being presented in equations ( 1) -(4): Fe 2+ + H 2 O 2 → Fe 3+ + HO -+ HO .
In the case of the chlorinated phenols, the number and the position on aromatic ring of the chlorine atoms modifies the oxidation efficiency [7,20].
* email: cristina27ccc@yahoo.com; Tel.: 0721259875 The oxidation rate constant decreases linearly with increasing number of chlorine content on the aromatic ring.Also, the increase of chlorine content will block some favorable positions susceptible to hydroxyl radical attack.
The oxidation process is also controlled by the presence of another species in reaction medium (intermediate products) in the sense that they interact with the catalyst component in a different manner.The species of reductive character accelerate the oxidation process because they reduce Fe 3+ (inactive) to Fe 2+ (active) and thus the generation of OH .radicals intensifies [21,22].The acid type species lower the pH of the reaction medium and can form stable complexes with Fe 3+ or Fe 2+ ions, strongly slowing the oxidation process [21].The presence into reaction medium of the inorganic ionic species (Cl -, ClO 4 -, NO ) modifies the rate of the oxidation of the organic compounds as function of their nature and concentration [21,[23][24][25][26][27][28][29][30][31].The inorganic anions can change the overall efficiency of the system by different ways.The influence of ClO 4 -and NO 3 -ions is less pronounced than another anions because they do not form complexes with Fe(II) and Fe(III) and do not react with HO .[25,27] [25,[27][28][29]31].The influence of Cl -is in correlation with the solution pH and its concentration, is insignificant at low concentration (<5 mM) but becomes very important at higher concentration values (>28 mM) [21].As function of the nature of the inorganic anions, at higher concentration of 0.1 M, the inhibition order of the oxidation rate is the following: . The objective of the present work is an experimental study on the influence of the ionic (NO 3 -, Cl -, SO 4 2-and PO 4 3-) and nonionic (bentonite) inorganic compounds on 4-chlorophenol (4-CP) oxidative degradation in aqueous solutions by advanced oxidation process photo-Fenton type. -

Experimental part
The laboratory experiments were achieved at room temperature (20±2 0 C) using a synthetic solution of 4chlorophenol equivalent concentrations of 300 mg O 2 /L COD value and the Fenton reagent (FeSO 4 and H 2 O 2 ) as oxidant agent.
The photo-catalytic reactor (fig. 1) leaves a ring space of 150 cm 3   The influence of the nature and concentration of the ionic species was achieved by natrium salts (chloride, nitrate, sulfate and phosphate) addition in the reaction medium to ensure predicted concentration of 0 to 2500 mg specific anion/L.
The influence of the insoluble species was achieved by bentonite addition in the reaction medium to ensure predicted turbidity of 0 to 150 nephelometric turbidity units (NTU), measured with UV-VIS Cintra 5 spectrophotometer (length wave domaine of 190-1100 nm).
The collected samples (MnO 2 addition for quickly unreacted H 2 O 2 decomposition and NaOH addition for Fe(OH) 3 precipitation) were filtered and analyzed quantitatively through a standard Chemical Oxygen Demand method (SR ISO 6060/1996).The quantitative evaluation of the 4-CP oxidation degree was performed by applying the relation: where: COD 0 is the initial Chemical Oxygen Demand (mg O 2 /L), COD is the final Chemical Oxygen Demand (mg O 2 / L) All reagents used in this work were analytical reagent grade from Aldrich.Hydrogen peroxide (30%, w/w, solution stock) was purchased from Fluka.

Influence of the inorganic ionic species
The experimental data concerning the influence of the inorganic ionic species Cl -, NO 3 -, SO 4 2-and PO 4 3-on the 4-CP oxidation efficiency (as COD and 4-CP oxidation degree The presence of the inorganic ionic species (Cl -, NO 3 -, SO 4 2-and PO 4 3-) into reaction medium influences the 4-CP oxidation efficiency by photo-Fenton process as function of their nature and concentration but in a different manner.
Nitrate ions induce a small influence on 4-CP oxidation efficiency.That may be explained by the absence of the interactions between NO 3 -and the catalyst (Fe 2+/3+ ) and hydroxyl radicals.NO 3 -will not produce a complex reaction with ferric ions, therefore, the reaction between ferric ions and hydrogen peroxide is not suppressed.NO 3 -will not react with hydroxyl radicals and the organic compounds oxidation is not inhibited.These results confirm the experimental data obtained by other authors [25,27].
The presences of Cl -, SO Into acidic solutions (pHd ≤ 3), phosphate primarily exists in the form of H 2 PO 4 -which will react with ferrous and ferric ions (reactions 14-15) to form inactive complexes [25,27]: As aresult, in the presence of Cl -, SO 4 2-and PO 4 3-ions, the concentration of the ferric and ferrous ions into reaction medium decreases and the oxidation efficiency is drastically affected.
Also, the Cl -, SO As a result, the concentration of hydroxyl radicals into reaction medium decreases, the oxidation process being affected.
At high concentration of SO 4 2-and PO 4 3-ions into solution, the precipitation reactions may be occurred (iron sulfate and phosphates solid phase forming) leading to a decrease of Fe 3+ concentration in solution and to diminish UV radiation effect.
The experimental data (fig.3) shows that the presence of inorganic ionic species Cl -, SO In conclusion, in photo-Fenton process, the presence of the inorganic anions into reaction medium influences the efficiency of the 4-CP oxidation as function of their nature as following:

Influence of the inorganic insoluble species
The presence of the insoluble inorganic species (bentonite) modifies the 4-CP oxidation efficiency in a different manner.Into reaction medium, 4-CP can be adsorbed by the bentonite substratum or can be destructed by oxidation, both processes increasing the 4-CP removal degree from the solution.
In the case of photo-Fenton process, the turbidity of the solution induced by the bentonite presence may affect the hydroxyl radicals generation process because the UV radiations intensity decreases by additional sorption process.As a result, the efficiency of the oxidation process of 4-CP is affected.
The experimental data concerning the cumulative adsorption and oxidation processes are presented in figures 4 and 5.
The experimental data for 4-CP removal by adsorption from solution ("Adsorption" dependence in Figs. 4 and 5), reported as COD value and 4-CP removal degree as function of the increasing turbidity of the solution (0-150 NTU), were obtained in absence of the Fenton reactive and UV radiations, at room temperature (20±2 0 C), using a synthetic solution of 4-chlorophenol equivalent concentrations of 300 mg O 2 /L COD value, initial solution pH 3±0.1 and 4-CP solution-bentonite contact time of 10 minutes.
The experimental data for 4-CP removal from solution, both by adsorption and oxidation process ("Oxidation and adsorption" dependence in figures 4 and 5), were obtained in similar conditions but in the presence of the Fenton reactive and UV radiations.
The data for 4-CP removal from solution by oxidation process only ("Oxidation" dependence in figures 4 and 5) were obtained by estimation from difference between the experimental data values for oxidation and adsorption cumulative processes and single adsorption process.
The experimental data presented in figures 4 and 5, obtained by photo-Fenton procedure in the presence of bentonite, show a slowly increasing of the removal of 4-CP with the increasing values of the turbidity of the solution.
At low concentration of the bentonite into reaction medium (turbidity values < 100 NTU), the oxidation process of 4-CP by photo-Fenton process is not significant affected.
At high concentration of the bentonite into solution (turbidity values > 100 NTU), the amount of 4-CP adsorbed increases but the oxidation process is affected.This may be explained by the decrease of the hydroxyl radical concentration into reaction medium as an effect of UV radiations intensity decreasing by additional bentonite sorption process.

Conclusions
The presence of the inorganic species inside the reaction medium influences the rate of the oxidation process as function of their nature and concentration.The inorganic anionic species reduce the 4-CP oxidation efficiency by Fe(II) and Fe(III) complexes forming, HO .radicals scavenging or iron precipitate forming.
NO 3 -induces a small influence on 4-CP oxidation efficiency.This may be explained by the absence of the interactions between NO 3 -and the catalyst (Fe 2+/3+ ) and hydroxyl radicals.
The anions Cl -, SO 4 2-and PO 4 3-modify drastically the 4-CP oxidation efficiency, especially at high concentration into reaction medium.They interact with Fe 2+ and Fe 3+ forming chloro-, sulfato-and phosphate-iron complexes  ) into reaction medium.The decrease of the 4-CP oxidation degree by the photo-Fenton procedure is correlated with the nature of the anions as following: Cl -> PO 4 3-> SO 4 2->> NO 3 -.The presence of the insoluble inorganic species (bentonite) modifies the 4-CP oxidation efficiency in a different manner.Into reaction medium, 4-CP can be adsorbed by the bentonite substratum or can be destructed by oxidation, both processes increasing the 4-CP removal degree from the solution.The presence of the insoluble inorganic species (bentonite) modifies the oxidation efficiency by additional 4-CP and UV sorption processes, especially at high solution turbidity values.
reaction volume (H = 410 mm, D = 33 mm, d=25 mm).The UV radiations are generated by a low pressure mercury vapors lamp, 40 W power. function on specified anion concentration into reaction medium) for 10 min reaction time, in specifies conditions, are presented in figures 2 and 3.The reference values, obtained in the absence of the inorganic ionic species after 10 min reaction time, are: the solution COD value 50 mg O 2 /L and 4-CP oxidation degree 83.3 %.

Fig. 1 .
Fig.1.Photo-catalytic reactor All experiments were accomplish in conditions of optimum basic parameters of the 4-CP oxidation process through the photo-Fenton procedure [32] respectively: initial 4-CP solution pH 3±0.1 (correction pH value with concentrate H 2 SO 4 and Jenway 370 pH-meter measured), H 2 O 2 (from stock solution, initial concentration determined by iodometric titration method) 28 mM concentration (to ensure a 50% excess compared to 4-CP/H 2 O 2 stoichiometric ratio), FeSO 4 .7H 2 O catalyst equivalent Fe 2+ concentration of 1.8 mM and 10 min reaction time.The influence of the nature and concentration of the ionic species was achieved by natrium salts (chloride, nitrate, sulfate and phosphate) addition in the reaction medium to ensure predicted concentration of 0 to 2500 mg specific anion/L.The influence of the insoluble species was achieved by bentonite addition in the reaction medium to ensure predicted turbidity of 0 to 150 nephelometric turbidity units (NTU), measured with UV-VIS Cintra 5 spectrophotometer (length wave domaine of 190-1100 nm).The collected samples (MnO 2 addition for quickly unreacted H 2 O 2 decomposition and NaOH addition for Fe(OH) 3 precipitation) were filtered and analyzed quantitatively through a standard Chemical Oxygen Demand method (SR ISO 6060/1996).The quantitative evaluation of the 4-CP oxidation degree was performed by applying the relation: