Scholarly article on topic 'Optimization, kinetics, physicochemical and ecotoxicity studies of Fenton oxidative remediation of hydrocarbons contaminated groundwater'

Optimization, kinetics, physicochemical and ecotoxicity studies of Fenton oxidative remediation of hydrocarbons contaminated groundwater Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — W.O. Medjor, O.N. Namessan, Eunice Adebowale Medjor

Abstract Fenton oxidation remediation of hydrocarbons contaminated groundwater was investigated for efficiency and effectiveness. 10% pollution was simulated in the laboratory by contaminating groundwater samples with diesel and domestic purpose kerosene (DPK) in two different experimental set ups. Optimum conditions of concentrations of the treatment solutions and pH were established: 300mg/L (FeSO4), 150,000mg/L (H2O2) and pH = 3 for the kerosene contaminant; 100mg/L (FeSO4), 300,000mg/L (H2O2) and pH = 3 for the diesel contaminant. The results from kinetics study show that the remediation process is pseudo-first order reaction with a rate constant of 8.07×104 mgL−1hr−1 and 3.13×104 mgL−1hr−1 for the diesel and kerosene contaminants in that order with 95.32% and 79.25% reduction in chemical oxygen demand (COD) for diesel and kerosene contaminated samples at the end of the remediation process respectively indicated that remediation have occurred significantly. Percent reduction in Total Petroleum Hydrocarbon (TPH) as kerosene was 89.84% and that of the diesel contaminant as 91.87% after 6hours of remediation. The general pollution index (GPI) for the hydrocarbons contaminated samples was in the range of 6.70–7.52 against the background value of 4.39 for the control groundwater sample. After treatment the GPI had dropped to 4.13–4.43 which depicts remarkable remediation although the samples remained impaired. Therefore there is the need of post-treatments to make the groundwater fit for domestic and agricultural uses. The application of the Fenton oxidative process is found to be very efficient, effective and rapid in reducing total petroleum hydrocarbon as kerosene and diesel as target contaminants.

Academic research paper on topic "Optimization, kinetics, physicochemical and ecotoxicity studies of Fenton oxidative remediation of hydrocarbons contaminated groundwater"

Egyptian Journal of Petroleum xxx (2017) xxx-xxx

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Optimization, kinetics, physicochemical and ecotoxicity studies of Fenton oxidative remediation of hydrocarbons contaminated groundwater

W.O. Medjor^*, O.N. Namessanb, Eunice Adebowale Medjorc

a Department of Chemistry, Taraba State University Jalingo, Nigeria

b Department of Agricultural Engineering and Bio-resources, Faculty of Engineering, Taraba State University Jalingo, Nigeria c Department of Civil Engineering, Faculty of Engineering, Taraba State University Jalingo, Nigeria

ARTICLE INFO ABSTRACT

Fenton oxidation remediation of hydrocarbons contaminated groundwater was investigated for efficiency and effectiveness. 10% pollution was simulated in the laboratory by contaminating groundwater samples with diesel and domestic purpose kerosene (DPK) in two different experimental set ups. Optimum conditions of concentrations of the treatment solutions and pH were established: 300 mg/L (FeSO4), 150,000 mg/L (H2O2) and pH = 3 for the kerosene contaminant; 100 mg/L (FeSO4), 300,000 mg/L (H2O2) and pH = 3 for the diesel contaminant. The results from kinetics study show that the remediation process is pseudo-first order reaction with a rate constant of 8.07 x 104 mgL 1hr 1 and 3.13 x 104mgL 1hr 1 for the diesel and kerosene contaminants in that order with 95.32% and 79.25% reduction in chemical oxygen demand (COD) for diesel and kerosene contaminated samples at the end of the remediation process respectively indicated that remediation have occurred significantly. Percent reduction in Total Petroleum Hydrocarbon (TPH) as kerosene was 89.84% and that of the diesel contaminant as 91.87% after 6 hours of remediation. The general pollution index (GPI) for the hydrocarbons contaminated samples was in the range of 6.70-7.52 against the background value of 4.39 for the control groundwater sample. After treatment the GPI had dropped to 4.13-4.43 which depicts remarkable remediation although the samples remained impaired. Therefore there is the need of post-treatments to make the groundwater fit for domestic and agricultural uses. The application of the Fenton oxidative process is found to be very efficient, effective and rapid in reducing total petroleum hydrocarbon as kerosene and diesel as target contaminants.

© 2017 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Article history: Received 12 February 2017 Revised 17 May 2017 Accepted 2 July 2017 Available online xxxx

Keywords:

Post-treatment

Pollution

Optimum

Rate constant

Pseudo-first order kinetics

1. Introduction

The world is over-dependent on crude oil. In spite of the advent of the new technology of production of alternative energy source from Shale oil (crude oil distilled from shale), massive activities of exploration, exploitation, production and transportation of crude oil have continued to be in the increase yearly across the globe. It has been predicted that the World use of petroleum and other liquid fuels would grow from 90 million barrels per day (b/ d) in 2012 to 100 million b/d in 2020 and to 121 million b/d in 2040. The petroleum and other liquid fuels act as primary feedstock for majority of petrochemical industries and also general

Peer review under responsibility of Egyptian Petroleum Research Institute.

* Corresponding author. E-mail address: weltime.medjor@yahoo.com (W.O. Medjor).

transport system [1]. Petroleum hydrocarbons represent highvolume global trade materials [1]. Chemically oil is known to consist mainly of aromatic and aliphatic compounds. These compounds are persistently toxic and difficult to deal with; particularly the benzene, toluene, ethyl benzene, xylene isomers (BTEX), and phenols [2]. In 2000 the world population was 6.2 billion. The United Nation has a projection of an additional 3 billion by 2050 with most of the growth from developing countries that are already experiencing water stress [3]. Groundwater and rivers form the main sources of water supplies that humans use (for domestic, industrial and agricultural uses). These several sources of water supplies are polluted by natural geological sources, pesticides, industrial discharge from various processing industries and oil spillage. As long as the longing urge for these global material persist, the unfortunate problems of pollution would remain with us and the quality of our environment remains largely compromised

http://dx.doi.org/10.1016/j.ejpe.2017.07.001

1110-0621/® 2017 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

W.O. Medjor et al./Egyptian Journal of Petroleum xxx (2017) xxx-xxx

and therefore leading to serious environmental problems [4]. The toxicity of the oil adversely affects the soil, plants and water resources. The devastating effects of pollution makes fresh water no longer useable without incurring painstaking high clean up costs [5].

Advanced oxidation process (AOP) via the use of hydrogen peroxide has been one of the most celebrated chemical remediation technologies in the treatment of petroleum refinery wastewaters to ameliorate the problems associated with pollution [6-11]. In most of the works cited in literatures optimum conditions of pH and concentrations; kinetics of remediation methods employed, physicochemical characterizations and ecotoxicity of samples after treatments are not usually discussed in details or referred to scantily. This present work is poised to investigate the optimum conditions, kinetics, physicochemical characterization and ecotoxicity of Fenton oxidation remediation of diesel and domestic purpose kerosene contaminated groundwater for efficiency and effectiveness.

2. Materials and methods

2.1. Materials

Automatic gasoline oil (diesel) and Domestic purpose kerosene (DPK) were obtained from Petroleum Marketers at Agbor, Delta State, Nigeria. All plastics and glassware used in the study were thoroughly pre-washed with detergent water solution, rinsed with tap water and soaked for 48 hours in 50% HNO3, then rinsed properly with deionised water and air-dried in the laboratory.

2.2. Preparation of samples

Pollution was simulated in the laboratory to mimic such occurrences in the natural environment. Seven plastic containers were each filled with 90 mL groundwater sample. 10 mL of kerosene was added to each of the containers and stirred thoroughly using a magnetic stirrer to obtain 10% contamination. These preparations were repeated in each experimental set up and for the diesel contaminant in replicates of five.

2.3. Preparations of treatment solutions

2.3.1. Preparation of hydrogen peroxide treatment solution for Fenton-oxidative method

Several concentrations of 50,000-500,000 mg/L of hydrogen peroxide were prepared by adding 5-50 mL of hydrogen peroxide solutions into several 100 mL volumetric flasks and made up to marks using distilled water.

2.3.2. Preparation of Iron(II) sulphate treatment solution for Fenton-oxidative method

Several concentrations of 100-700 mg/L of iron(ll) sulphate solutions were prepared by dissolving 100-700 mg of iron(ll) sulphate in seven different volumetric flasks (1000 mL), thoroughly shaken and made up to marks using distilled water.

2.3.3. Optimization of concentration of treatment solutions and pH Optimization of concentration of hydrogen peroxide, iron(ll)

suulphate and pH of treatment solutions were determined to ascertain the optimum conditions for TPH removal for each of the two contaminants employed in the study.

2.3.4. Optimization of concentration of hydrogen peroxide on Fenton-oxidative method

TPH as kerosene was determined by molecular spectrophotom-etry following standard procedure adopted by Wang et al. [12]

with slight modification of drying the wet extracts with anhydrous sodium sulphate to avoid unnecessary interference of impurities. Several solutions of the 10% kerosene contaminated groundwater taken in seven conical flasks were added to each 6 mL of 300 mg/ L FeSO4 and 30 mL of 5-35% wt H2O2 and allowed to undergo remediation for 30 minutes before extraction and analysis. Kerosene in the water layers was extracted using hexane. The wet extracts were passed through 5 g of anhydrous sodium sulphate on Number 42 Whatmann filter paper in different filtration set ups in order to obtain clean and dry extracts. TPH as kerosene was read off by UV/Visible spectrophotometer (made in the United Kingdom, 2007 model) at wavelength of 310 nm. The procedure was repeated using the other four replicates samples. Experiment was repeated using diesel contaminated groundwater samples at wavelength of 350 nm.

2.3.5. Optimization of concentration of FeSO4 on Fenton-oxidative method

The optimum concentration of hydrogen peroxide obtained in this same study was used to determine the optimum concentration of iron(ll) sulphate. The 30 mL of the optimum concentration of hydrogen peroxide was added to 6 mL of several concentrations of FeSO4 (50-700 mg/L) in eight conical flasks; to each flask was added 10% kerosene contaminated groundwater sample and allowed to undergo remediation for 30 minutes before extraction and analysis. Kerosene in the water layers were extracted using hexane. TPH as kerosene were determined standard method adopted by Wang et al. [12] and read off at wavelength of 310 nm using T - 60 UV/Visible spectrophotometer (made in United Kingdom, 2007 model). Experiment was repeated using diesel contaminated groundwater samples at wavelength of 350 nm.

2.4. Kinetics studies on Fenton-oxidative method

The pH of the domestic purpose kerosene of contaminated groundwater samples were adjusted to pH = 3 and treated with concentrations of 150,000 mg/L of hydrogen peroxide and 300 mg/L of iron(ll) sulphate obtained from the optimization study in this same study and allowed to undergo remediation at an hour interval for 6 hours and extraction and analysis was carried out at an hourly interval . Kerosene in the water layer was extracted using hexane and TPH as kerosene were determined by UV/Visible spectrophotometer at wavelength of 310 nm using standard method adopted by Wang et al. [12] with modifications described in the same work.

The concentrations of TPH as kerosene left were plotted against time. Typical pseudo-first-order rate plot for the remediation process was tested by plotting Ln[B]o - Ln[B]t against time using the experimental kinetics data. The slope of the graph is the first order rate constant, k. The actual pseudo-first order rate constant was obtained by multiplying the value of the slope from the graph with the optimum concentration of the treatment solution that was used in excess, [A]o in the optimisation experiment in this same work. Experiment was repeated using diesel contaminated ground-water samples at wavelength of 350 nm with pH of the solutions adjusted to pH = 3; 300 mg/L of iron(ll) sulphate and 300,000 mg/ L of H2O2.

2.5. Controlled experiments

Two plastic containers were each filled with 90 mL groundwa-ter sample with known background hydrocarbon level for each of the contaminant. 10 mL of diesel and 10 mL of kerosene was added to each of the set ups to produce 10% contamination. The simulated control samples without the Fenton oxidation treatment solutions were left to undergo remediation in the laboratory for an optimum

W.O. Medjor et al./Egyptian Journal of Petroleum xxx (2017) xxx-xxx

time of 6 hours. Extractions and analysis for TPH followed using standard procedures already described in the same work.

2.6. Determination of physicochemical properties of samples

Physicochemical parameters such as pH, turbidity, alkalinity, dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), electrical condutiivity (EC), ammonia, nitrate, phosphate, salinity, total dissolved solids (TDS), total suspended solids and total solids were obtained using standard methods adopted by APA [13].

Heavy metal contents were determined using atomic absorption spectrophotometer (AAS) following procedures adopted by Nabil and Barbara [14]. Sample was first digested using the standard method proposed by APA [13]. 50 mL of the sample was treated with 5 mL of concentrated HNO3 and heated on a hot plate with gradual addition of concentrated HNO3 as necessary until the solution boiled in a fume cupboard. It was then evaporated to about 20 mL; 5 mL of concentrated HNO3 was finally added, covered and allowed to cool and then filtered. The filtrate was poured into a 50 mL standard volumetric flask and made up to the mark with distilled water. Portion of the solution was used for heavy metals (Cd, Cr, Ni, Pb, Hg, V, Se and As) determinations.

2.7. Ecotoxicity test

Ecotoxicity tests were performed to determine the potential contamination of the environmental samples used in the study. Five pollution indices were calculated to effectively workout a value that would unite all the contamination parameters found in the environmental sample following procedures adopted by Boluda et al. [15]. Pollution indices considered in the study were:

(i) PPI = ^H/9+Alkalinity(mg/L)/45+EC(dS/m)/3+TDS(mg/L)/1440j

(ii) SI = ^EC(dS/m)/3+ Chloride(mg/L)/335j

(iii) OP1 = [COD (mg/L)/30 + BOD (mg/L)/6) + DO (mg/L)/4) + Chloride (mg/L)/335 + NH4-N (mg/L)/1 + NO3-N (mg/ L)/50)]/6

(iv) CP1HM = [Cr (mg/L)/0.05 + Ni (mg/L)/0.1 + Pb (mg/L)/0.05 + Cd (mg/L)/0.005]/4

(v) GP1 = [PPI + SI + OP1 + CP1HM]/4

where: PP1 = physical pollution index; S1 = salinity index; OP1 = organic pollution index; CP1HM = chemical pollution index for heavy metals; GP1 = general pollution index.

2.8. Statistical analysis

64 63.5 63 62.5

o 62 ra 61.5 ^ 61 S 60.5 60 59.5 59 58.5

0 100000 200000 300000 400000 500000 600000 Concentration of hydrogen perioxide (mg/L

Fig. 1. Plot of % remediation of TPH against concentrations of hydrogen peroxide for the optimization study of diesel contaminated groundwater using Fenton oxidative method.

o 62.8

■■P

S "o 62.6

J 62.4 62.2 62 61.8

0 100 200 300 400 500 600 700 800 Concentration of iron(II)sulphate(mg/l)

Fig. 2. Plot of % remediation of TPH against concentrations of iron(II) sulphate for the optimization study of diesel contaminated groundwater using Fenton oxidative method.

tions of 150,000 mg/L H2O2 and 300 mg/L FeSO4 solutions were obtained for the kerosene contaminated groundwater samples with 49.67% remediation efficiency. The results are shown in Figs. 3 and 4.

The mixture of the hydrogen peroxide and iron(II) sulphate solutions used in the study is acidic in nature (pH = 4.43). It was found that when the pH of the reacting mixture was adjusted, remediation is slightly more favourable at pH = 3 (71.65%) than pH = 8 (71.52%) for the diesel contaminant and also more favourable at pH = 3 (61.79%) than at pH = 8 (61.67%) for the kerosene contaminant.

In similar studies in literature suggest that remediation is more effective and efficient at pH range of 3-6 for the Fenton-oxidative

Statistical analyses were executed with the aid of Microsoft Office Excel 2007 and SPSS Version 16. Standard deviations were determined to checkmate indeterminate errors. Also analysis of variance (ANOVA) was carried out at P < 0.05 on the means of TPH from the kinetics study for the two contaminants to determine if there was any significant difference in TPH at the end of remediation in the different treatments.

3. Results and discussion

The results of optimization of concentration for hydrogen peroxide, iron(II) sulphate as catalyst for diesel contaminated ground-water samples treated by Fenton oxidative method are represented in Figs. 1 and 2. Diesel contaminated groundwater samples treated with 300,000 mg/L H2O2 and 100 mg/L FeSO4 solutions had an optimum remediation efficiency of 63.2%. Optimum concentra-

50 49.5 49

•S 48.5

47.5 47

46.5 -I-1-1-1-1-1-1-1-1

0 50000 100000 150000 200000 250000 300000 350000 400000 Concentration of hydrogen perioxide (mg/l)

Fig. 3. Plot of % remediation of TPH against concentration of hydrogen peroxide solution for the optimization study of kerosene contaminated groundwater using Fenton oxidative method.

W.O. Medjor et al./Egyptian Journal of Petroleum xxx (2017) xxx-xxx

Si: 30

200 400 600

Concentration of iron(II)sulphate (mg/l)

Fig. 4. Plot of % remediation of TPH against concentration of iron(II) sulphate for the optimization study of kerosene contaminated groundwater using Fenton oxidative method.

method [16]. The potency of a strong oxidant such as hydrogen peroxide has been reported to enhance the reducing conditions typical of low pH [17]. This assertion supports the optimum pH of 3 obtained in this study.

The preliminary kinetics profiles of Fenton oxidative method for the diesel and kerosene contaminants are shown in Fig. 5. It was observed that TPH decreased with time in all cases. This is a strong indication that there are interactions between total petroleum hydrocarbon molecules and the reacting species of the treatment solution for the different contaminant employed in the study. The results of correlation statistics of -0.770 for the kerosene and -0.779 for the diesel contaminants at p < 0.05 also indicate that there is substantive negative correlation between TPH and time. The correlation statistics therefore further supports the observed decrease in TPH with time. The results from the progress profile of the typical pseudo-first-order rates are represented in Fig. 6. The plots show good linearity and regression coefficient (R2) of 0.995 for the diesel contaminant and 0.9158 for the kerosene contaminant. The kinetics data of each of the contaminant adequately fits into the kinetics model that satisfies the conditions for pseudofirst order reaction. The actual pseudo-first order rate constants for the diesel and kerosene contaminated groundwater were calculated as 8.07 x 104mgL-1hr-1 and 3.13 x 104mgL-1hr-1 in that order and consistent with works of EPA [18] on kinetics of Fenton reagent on contaminant of common concern where a pseudo first order reaction has also been reported. The result from the percent reduction in total petroleum hydrocarbon as kerosene was 89.84% and that of the diesel contaminant as 91.87%. The results analysis of variance (ANOVA) on the means of TPH obtained for diesel and

8000 7000 6000 _ sooo

m(4000

^ 3000 2000 1000 0

DIESEL KERO

01234567 Time/hr

Fig. 5. Progress profile of TPH as diesel and kerosene against time in Fenton oxidative method.

о 1.5

♦ DIESEL KERO

01234567 Time/hours

Fig. 6. Progress profile of Pseudo - first order plots for diesel and kerosene in Fenton oxidative method.

kerosene indicates that there is no significant difference in TPH reduction for the two contaminants; F(6, 7) = 20.928, P = 0.000. An earlier assertion inferred by the percent statistic. This implies that the Fenton oxidative process is equivocally effective and efficient for the remediation of diesel or kerosene contaminated environmental samples. The TPH reduction in the controlled experiments was in the range of 1.3-1.5% which is considered to be negligibly small and due evaporation and microbial actives. The outcome from the controlled experiments and ANOVA show that the reduction in TPH observed in the work is as a result of the Fen-ton oxidative treatments and not any other confounding variables; temperature and other physical conditions being kept constant.

The results from the physicochemical characterization of the uncontaminated groundwater sample, hydrocarbons contaminated groundwater samples before remediation and after remediation are presented in Table 2. The pH of the diesel and kerosene contaminated groundwater samples was 9.50 and 9.40 in that order while that of the uncontaminated groundwater sample was 8.60 ± 0.06. After treatment there is a slight drop in pH to 9.4 for the diesel contaminated samples and a slight increase for the kerosene contaminated samples. Both values are not within the permissible limit set for pH by FMEnv. [19]. Although pH usually has no direct impact on consumers, it is one of the most important operational water quality parameter [20].

The turbidity of the uncontaminated groundwater sample was 0.2 ± 0.00 NTU while the samples contaminated with diesel and kerosene had turbidity values that were very high (856 ± 2.34 NTU) for diesel; (1124 ± 5.78 NTU) for kerosene. These values are astronomically high and an indicator that the prepared groundwater samples are highly polluted compared to 100275 NTU turbidity values for highly polluted water reported by Rump [21]. After remediation although there is sporadic reduction in the turbidity levels but only the diesel contaminated samples had turbidity value (1.90 ± 0.02 NTU) that are within the limit (5.00 NTU) set by FMEnv. [19] for turbidity (Table 1). The turbidity level for the kerosene treated samples were somewhat high (130 ±0.000 NTU) and above the limit (5.00 NTU) set by FMEnv. [19] for turbidity. The removal of particulate matter by coagulation, sedimentation and by filtration is thus required in achieving safe drinking-water [20].

The electrical conductivity of the uncontaminated groundwater sample and diesel contaminated samples had same value of 100 ± 0.000 iS/cm but that of the kerosene contaminated samples had value of 300 ± 0.000 iS/cm. These values are within the limit allowed for conductivity by FMEnv. [19] (Table 1). These low electrical conductivity values for the hydrocarbon contaminated groundwater samples are likely due to non-polar environments provided by the hydrocarbons that help in retarding the

W.O. Medjor et al./Egyptian Journal of Petroleum xxx (2017) xxx-xxx 5

Table 1

Analysis of variance (ANOVA) on the TPH values for diesel and kerosene obtained from the kinetics studies in the Fenton oxidative method employed in the study.

VAR00001

Sum of squares df Mean square F Sig.

Between groups 4.321E7 6 7201142.001 20.928 0.000

Within groups 2408690.228 7 344098.604

Total 4.562E7 13

P < 0.05.

movements and immobilizing of ions present in the solutions. The resultant effect is reduction in ionic mobility, velocity and electrical conductivity. After remediation the conductivity of the treated samples was 300 iS/cm and within the limit of 1200 iS/cm permissible level for conductivity set by FMEnv. [19].

The uncontaminated groundwater sample had an alkalinity concentration of 148 mg/L while the diesel and kerosene contaminated groundwater samples had values of 132 mg/L and 136 mg/ L in that order as shown in Table 1. The decrease in the alkalinity values recorded after contamination is likely due to chemical interaction between the acidic components of the contaminants and the carbonates or hydroxyl ions present in the groundwater sample. After treatments alkalinity value for the diesel samples was 68.00 ± 0.000 mg/L and 88.00 ± 1.44 mg/L for the kerosene samples. These values are within the standard value of 100 mg/L allowed by FMEnv. [19]. A plausible explanation for this observation is that the highly reactive Fenton reagent (hydrogen peroxide and iron(II) sulphate must have decomposed the carbonates present in the groundwater samples to CO2 and H2O.

The dissolved oxygen (DO) value of the uncontaminated groundwater sample was 7.30 mg/L while those of diesel and kerosene contaminated groundwater samples had values of 3.55 and 3.20 mg/L respectively as shown in Table 1. The drop in the dissolved oxygen values after the groundwater samples were simulated with the contaminants indicates pollution. Less microbial activities depicts high value of DO for the diesel contaminated groundwater samples compared with those of kerosene samples. The treated samples had DO value in the range of 5.79-6.92 mg/ L. This indicates significant improvements compared with the 7.30 mg/L of the uncontaminated groundwater sample and within the 15 mg/L limit permissible allowed by FMEnv. [19]. This increase in DO is associated with the decomposition of the hydrogen peroxide to form water and oxygen. It is also possible that the release of oxygen from the Fenton reagent system stimulated aerobic biological activities and further enhanced the effectiveness of the Fenton oxidative process.

The biochemical oxygen at day-5 (BOD5) for the uncontami-nated sample was 2.8 ± 0.133 mg/L on contamination the value had increased to 7.39 ± 0.133 mg/L and 4.29 ± 0.133 mg/L diesel and kerosene samples in that order. The BOD5 for the treated contaminated samples was in the range of 4.09-6.49 mg/L. The sharp drop in BOD5 value for treated samples compare to those of the polluted samples also corroborates the fact that remediation have taken at place to a significant varying degrees [22], [23]. The observed BOD5 values are within the limit of 7.5 mg/L set by FMEnv. [19].

Chemical oxygen demand (COD) value for the uncontaminated sample was 524.00 ± 0.01 mg/L and on contamination the value had decreased to 270.00 ± 0.01 mg/L for diesel samples and an increase to 668.00 ± 0.05 mg/L for the kerosene samples. The COD being a measure of the total oxidizable organic matter is expected to increase on addition of the contaminants. The observed COD drop for diesel contaminated groundwater samples is due to some aromatic hydrocarbons, straight-chain aliphatic and nitrogeneous compounds present in diesel that are not readily oxidizable. After

remediation the value of COD of the treated contaminated samples had drastically dropped to 12.61 ± 0.01 mg/L and 138.63 ±3.34 mg/L for diesel and kerosene samples respectively; an indicator that remediation have occurred significantly but these values are far higher than the 10 mg/L limit set by FMEnv. [19]. The low ratio of COD/BOD5 1.94 for the diesel contaminated samples and 33.90 for the kerosene contaminated samples after remediation compared with that of the polluted samples (36.54-155.71) with low concentration of oxidizable organic matter is indicative that remediation have occur. The significant reduction in COD 95.32% and 79.25% for the diesel and kerosene contaminated samples respectively indicates enhanced remediation.

The salinity level for the diesel and kerosene contaminated groundwater samples was in the range 60.29-188.99 mg/L while the uncontaminated groundwater sample had a value of 103.19 ± 0.000 mg/L. Salinity level of the treated samples was 188.99 mg/L and within the 1600 mg/L recommended limit by FMEnv. [19].

The uncontaminated groundwater sample had nitrate concentration of 148.00 ± 3.44 mg/L while those of diesel and kerosene contaminated groundwater samples had value of 16 ± 0.010 mg/L and 34 ± 1.22 mg/L in that order as shown in Table 1. The high value of nitrate in kerosene contaminated groundwater samples is owned to less pronounced oxidative action of the acidic components in the kerosene constituents. After remediation the treated samples were found to have nitrate levels in the range of 1017 mg/L and within the 50 mg/L limit set by FMEnv. [19].

The phosphate concentration in the uncontaminated water sample was 0.28 mg/L while those of diesel and kerosene contaminated water samples had concentrations of 0.16, 0.34 mg/L respectively. There was decrease in the phosphate levels in the diesel contaminated groundwater samples compared with that of the control groundwater sample while the values for kerosene contaminated groundwater samples were slightly higher (Table 1). All the contaminated samples have values for phosphate that are not within the permissible limit set by FMEnv. [19] even after remediation.

The uncontaminated groundwater sample had ammonia content of 0.53 ± 0.001 mg/L but on contamination with the hydrocarbons, a range of 0.42-0.89 mg/L was obtained. The treated samples contaminated diesel were found to have value of 0.15 mg/L for ammonia and within the safe limit of 0.5 mg/L allowed by FMEnv. [19]. Ammonia was beyond the level of detection in the kerosene contaminated samples. Ammonia in drinking - water is not of immediate health relevance. However, ammonia can compromise disinfection efficiency, result in nitrite formation in distribution systems, causes the failure of filters for the removal of manganese and cause taste and odour problems [20].

The total suspended solids (TSS) level for the uncontaminated groundwater sample was 1.089 ± 0.003 mg/L while diesel and kerosene contaminated water samples had value of 1.383 ± 0.024, and 0.85 ± 0.041 mg/L respectively (Table 1). The introduction of diesel into the groundwater samples made the mediums to have nonpolar hydrophobic environment and decreased solubility for solutes present and is responsible for the high value of TSS in comparison with the value of uncontaminated groundwater sample.

W.O. Medjor et al./Egyptian Journal of Petroleum xxx (2017) xxx-xxx

The TSS value for the kerosene contaminated groundwater samples were low compared with the value for the uncontaminated groundwater sample. A plausible explanation to this could be that other kinds of molecular interactions as well as lesser non-polar-hydrophobic medium provided by the kerosene contaminant having shorter hydrophobic chain length than the diesel contaminant. This may certainly have led to the increased solubilisation of the solutes in the water-kerosene interface and decreased amount of TSS. After remediation there is significant increase in TSS in the hydrocarbons treated samples. All the treated hydrocarbons samples have TSS level within the permissible limit of 10 mg/L set by FMEnv. [19] forTSS.

The concentration of the total dissolved solids (TDS) in the uncontaminated groundwater sample was 115 mg/L while those of the contaminated groundwater samples ranged from 160 to 170mg/L. After remediation the treated samples had TDS value of 130 mg/L. The Fenton reagent consisted of mainly hydrogen peroxide solution and small concentration of iron(ll) tetraoxosulphate (Vl) salt therefore responsible for the low concentration of TDS for the treated samples obtained coupled with the oxidative nature of the process. These values are within the 500 mg/L safe limit set for TDS by FMEnv [19]. However, the presence of high level of TDS in drinking-water may be objectionable to consumers and may affect acceptability of drinking - water [20].

The total solids ((TS) level in all the treated samples was observed to be low (130 mg/L) after remediation. This is excepted since the TDS value obtained in the same work was low and total solid is the sum of TSS and TDS. Total solid values for all the treated samples are within the 1500 mg/L value for total solids allowed by FMEnv. [19].

Some heavy metals such as lead (Pb), vanadium (V) and selenium (Se) were beyond the level of detection in the uncontami-nated groundwater sample and diesel contaminated groundwater samples. The kerosene contaminated samples had level of

0.40 ± 0.001 mg/L for Pb, but the levels of V and Se were also beyond the level of detection. After remediation the levels of Pb and Se were beyond detectable level in all the treated samples. Vanadium metal had level of 0.008 ± 0.0001 mg/L for diesel and kerosene treated samples. This value is within the safe limit of 0.05 mg/L allowed by FMEnv. [19] for vanadium. Vanadium metal was neither found in the control groundwater sample nor in the hydrocarbons contaminated samples suggests that the vanadium metal must have been introduced from the treatment solution.

Heavy metals levels increased significantly on contamination with hydrocarbons (Table 2). After remediation 44.4% reduction (Cd); 98.4% reduction (Cr); 54.2% increment (Ni); reduction 52.38% (Hg); 1400% increment (As) were observed for treated diesel contaminated samples. For the kerosene contaminated treated samples reduction and increment of the metals after remediation were: 36.4% reduction (Cd); 85% reduction (Cr); 171.4% increment (Ni); 27.6% reduction (Hg); 75% reduction (As). The heavy metals levels of Cd, Ni, and Hg, in the hydrocarbons contaminated groundwater samples are higher than the limits set for these metals by FMENv [19]. Therefore all the treated hydrocarbons contaminated samples needed post treatments after remediation in terms of these heavy metals. Chromium level in the treated contaminated diesel samples and arsenic level in the treated contaminated kerosene samples are within the safe limit set by FMEnv. [19].

The results from the ecotoxicity studies show that the uncon-taminated groundwater and hydrocarbon simulated groundwater samples had a physical pollution index (PPl) in the range of 1.02-1.09 and an indication of pollution. Pollution index (Pl) in the range of 1-2 has been cited in literature to indicate slight pollution [24,25]. After treatments the range had dropped to 0.66-0.8 suggests that the treated samples are free from pollution; as Pl < 1 indicates no pollution as also reported by Caerio et al. and Amadi et al. [24,25] in their studies.

Table 2

Results of physicochemical characterization and ecotoxicity of uncontaminated groundwater sample, contaminated samples and treated samples used in the study.

Parameter Uncontaminated groundwater Sample Diesel + groundwater Sample Kerosene + groundwater Sample Treated diesel samples by Fenton Oxidation Treated Kerosene Samples by Fenton Oxidation FMENV (1991)

pH 8.60 ± 0.06 9.50 ± 0.000 9.40 ± 0.050 9.40 ± 0.1 9.50 ± 0.01 9.2

Conductivity (mS/cm) 100 ±0.010 100 ±0.000 300 ±0.010 300 ± 0.00 300 ± 0.00 1200

Turbidity (NTU) 0.2 ± 0.000 856 ± 2.340 1124 ±5.780 1.90 ±0.000 130.0 ±0.000 5

Alkalinity as CaCO3 (mg/L) 148.00 ± 0.01 132.00 ±0.000 136.00 ±0.010 68.00 ± 0.00 88.00 ±1.44 100

Dissolved Oxygen (mg/L) 7.29 ±0.133 3.55 ± 0.058 3.19 ±0.000 6.92 ± 0.066 5.79 ± 0.066 15

BOD5 (mg/L) 2.80 ±0.133 7.39 ± 0.066 4.29 ± 0.000 6.49 ±0.114 4.09 ±0.116 7.5

COD (mg/L) 524.00 ±0.010 270.00 ±0.010 668.00 ± 0.050 12.61 ±0.01 138.63 ±3.34 10

COD/BOD5 - 36.54 155.71 1.94 33.90 -

Reduction in COD - - - 95.32 79.25

Salinity as chloride (mg/L) 103.19 ±0.000 103.19 ±0.000 188.99 ±0.000 188.99 ±0.000 188.99 ±0.000 1600

Nitrate (mg/L) 148.00 ± 3.44 16.00 ±0.010 34.00 ± 1.220 17.00 ±0.01 10.00 ±0.001 50

Phosphate (mg/L) 0.28 ± 0.001 0.16 ±0.004 0.34 ± 0.008 0.81 ± 0.0003 0.66 ± 0.0004 0.02

Ammonia (mg/L) 0.53 ± 0.001 0.89 ± 0.005 0.42 ± 0.001 0.15 ±0.0003 ND 0.5

Total dissolved solids (mg/L) 115.00 ±8.086 170.00 ±0.000 160.00 ±0.000 130.0 ±0.00 130.0 ±0.00 500

Total suspended solids (mg/L) 1.089 ± 0.003 1.383 ±0.024 0.846 ± 0.041 2.237 ± 0.052 1.386 ±0.015 10

Total solids (mg/L) 116.09 ± 7.76 171.38 ±5.55 160.85 ±0.01 132.24 ±1.630 131.39 ±0.001 1500

Cadmium (mg/L) 0.21 ± 0.001 0.45 ± 0.0004 0.33 ± 0.001 0.25 ± 0.0006 0.21 ± 0.0005 0.001

Chromium (mg/L) 0.10 ±0.000 0.60 ± 0.006 0.40 ± 0.001 0.01 ± 0.006 0.06 ± 0.0005 0.05

Nickel (mg/L) 0.62 ± 0.0003 0.59 ± 0.0006 0.28 ± 0.001 0.91 ± 0.0005 0.76 ± 0.0006 0.02

Lead (mg/L) ND ND 0.40 ± 0.001 ND ND 0.05

Mercury (mg/L) 0.37 ± 0.0006 0.42 ± 0.0006 0.29 ± 0.0001 0.22 ± 0.01 0.21 ± 0.001 0.02

Vanadium (mg/L) ND ND ND 0.008 ± 0.00003 0.008 ± 0.0001 0.05

Selenium (mg/L) ND ND ND <0.001 0.012 ±0.00003 0.05

Arsenic (mg/L) 0.03 ± 0.0006 0.04 ± 0.0005 0.04 ± 0.0006 0.60 ± 0.0004 0.01 ± 0.0006 0.05

TPH (mg/L) 22.00-30.00 5199.26 ±3.770 6896.22 ± 3.670 422.48 ± 1.630 700.45 ± 4.25 10

PPI 1.09 1.02 1.07 0.66 0.80 -

SI 0.02 0.02 0.33 0.33 0.33 -

OPI 3.92 2.11 4.24 0.71 1.17 -

CPIHM 12.55 26.98 21.20 14.83 15.40 -

GPI 4.39 7.52 6.70 4.13 4.43 -

W.O. Medjor et al./Egyptian Journal of Petroleum xxx (2017) xxx-xxx

4. Conclusion

The controlled experiments provided high internal validity for the study; therefore relationships between time, pH, concentrations and reductions in TPH were glaring explored. The study established optimum conditions of pH, contact time and concentrations of hydrogen peroxide and iron(II) sulphate for the Fenton oxidative process for each of the target contaminants. The application of the Fenton oxidative process is found to be very efficient, effective and rapid in reducing total petroleum hydrocarbon as kerosene and diesel as target contaminants. 63% of the physico-chemical parameters investigated met the Guideline and Standards for Environmental Pollution Control in Nigeria set by Federal Ministry of Environment and those of the World Health Organisation for drinking water/agricultural uses. Yet there is the need for post-treatments after remediation to make the groundwater fit for domestic and agricultural uses since 37% of the physicochemi-cal parameters were found to be impaired.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Competing interest

Authors have declared that no competing interests exist. References

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