Decolorization and Organic Removal from Palm Oil Mill Effluent by Fenton's Process

Abstract

Decolorization and organics removal in palm oil mill effluent (POME) were investigated with the Fenton process. Decolorization efficiency obtained was 66.5% at 5 min and >90.0% at 30 min of reaction time. The process removed 82.0% of soluble chemical oxygen demand at 1 h of reaction time with optimized conditions: [H₂O₂]=50 mM, [Fe²⁺]=1.0 mM, and pH=3.0±0.2. The kinetics rate constants (k) of decolorization and dissolved organic carbon removal were found at 6.20×10⁻² min⁻¹ and 2.19×10⁻⁴ (mg/L)⁻¹ min⁻¹, respectively. However, the UV irradiation did not obviously enhance the decolorization and organic removal of POME. Humic and fulvic acid-like substances were identified at Ex/Em of 315/415 and 260/445 nm, respectively. By fluorescence excitation-emission matrix (FEEM), spectroscopy could create colors in POME. Reduction of FEEM intensity was 98.61%, which was related to the decolorization.

Introduction

Oil Palm (Elaeis guineensis) is one of the most common economic crops in tropical regions. It is a very important agricultural-based industry, notably in Southern Thailand, Malaysia, and Indonesia. Extraction of palm oil involves a number of processes that require large quantities of water. It usually requires an estimated 1.5 m³ of water to process a ton of fresh fruit bunch, and half of this amount becomes palm oil mill wastewater (POMW). Raw POMW is a thick brownish liquid discharged at a high temperature (80°C–90°C) and contains organic pigments, such as anthocyanins, carotene, polyphenols, lignin, and tannin. It is characterized by a low pH of 3.5–5.0, a chemical oxygen demand (COD) of 40,000–50,000 mg L⁻¹, a biochemical oxygen demand (BOD₅) of 20,000–30,000 mg L⁻¹, and a high total suspended solids (TSS) of 5000–54,000 mg L⁻¹ (Ahmad et al., 2003).

Biodegradation system is the most popular method used for POMW treatment, but it does not always decolorize effectively that result nondischargeable palm oil mill effluent (POME). Therefore, the physical-chemical methods such as advanced oxidation processes (AOPs), separation, precipitation, and adsorption processes are being considered in further decolorization of the organic pigments. The AOPs offer greater efficiency due to their ability to diminish various pollutants such as aniline, phenol, and dimethyl sulfoxide in the wastewater (Li et al., 2010; Huang et al., 2011a, 2011b). They are based on the generation of hydroxyl radicals (•OH) that have powerful oxidizing capabilities. Among the AOPs, Fenton's reagent is an attractive one, because the ferrous ions (Fe²⁺) and hydrogen peroxide (H₂O₂) are widely available and easy to handle, and excess decomposition is not harmful to the environment. The Fenton process involves several sequential reaction steps as follows (Yoon et al., 2001): \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} { \rm Fe}^{2 + } + { \rm H}_2{ \rm O}_2 \rightarrow { \rm Fe}^{3 + } + { \rm OH}^ - + \bullet { \rm OH} \ k_1 = ( 53 - 76 )\ { \rm M}^{ - 1} \ { \rm s}^{ - 1} \tag { 1 } \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} { \rm Fe}^{2 + } + \bullet { \rm OH} \rightarrow { \rm Fe}^{3 + } + { \rm OH}^ - k_2 = ( 3.2 - 4.3 ) \times 10^8 \ { \rm M}^{ - 1} \ { \rm s}^{ - 1} \tag { 2 } \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} \bullet { \rm OH} + { \rm Org}. \rightarrow { \rm Org.}\ { \rm Org} \ldots \rightarrow { \rm CO}_2 + { \rm H}_2{ \rm O} \ k_3 \approx 10^7 \ - 10^{10} \ { \rm M}^{ - 1} \ { \rm s}^{ - 1} \tag { 3 } \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} { \rm H}_2{ \rm O}_2 + \bullet { \rm OH} \rightarrow { \rm H}_2{ \rm O} + { \rm HO}_2 \ k_4 = ( 1.2 - 4.5 ) \times 10^7 \ { \rm M}^{ - 1} \ {\rm s}^{ - 1} \tag { 4 } \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} \bullet { \rm OH} + \bullet { \rm OH} \rightarrow { \rm H}_2{ \rm O}_2 \ k_5 = 5.3 \times 10^9 \ { \rm M}^{ - 1} \ { \rm s}^{ - 1} \tag { 5 } \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} { \rm Fe}^{3 +} + { \rm H}_2{ \rm O}_2 \rightarrow \hbox{Fe - OOH}^{2 + } + { \rm H}^ + \ k_6 = 0.001 - 0.020 \ { \rm M}^{ - 1} \ { \rm s}^{ - 1} \tag { 6 } \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} { \rm Fe}^{2 + } + ^{\bullet}{ \rm HO}_2 + { \rm H}^ + \rightarrow { \rm Fe}^{3 + } + { \rm H}_2{ \rm O}_2 \ k_7 = 1.2 \times 10^6 \ { \rm M}^{ - 1} \ { \rm s}^{ - 1} \tag { 7 } \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} { \rm Fe}^{3 + } + ^{\bullet}{ \rm HO}_2 \rightarrow { \rm Fe}^{2 + } + { \rm O}_2 + { \rm H}^ - \ k_8 = 1.0 \times 10^4 \ { \rm M}^{ - 1} \ { \rm s}^{ - 1} \tag { 8 } \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} { \rm H}_2{ \rm O}_2 + ^{\bullet}{ \rm HO}_2 \rightarrow \bullet { \rm OH} + { \rm H}_2{ \rm O} + { \rm O}_2 \ k_9 = 0.5 \pm 0.09 \ { \rm M}^{ - 1} \ { \rm s}^{ - 1} \tag { 9 } \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} \bullet { \rm OH} + ^{\bullet}{ \rm HO}_2 \rightarrow { \rm H}_2{ \rm O} + { \rm O}_2 \ k_{10} = 1.0 \times 10^{10} \ { \rm M}^{ - 1} \ { \rm s}^{ - 1} \tag {10} \end{align*} \end{document}

This process has shown a promising applicable prospect in the treatment of several kinds of organic or dye pigments contaminating wastewaters, including those discharges from livestock (Lee and Shoda, 2008), azo dye (Meriç et al., 2004) and olive oil mills (Ugurlu and Kula, 2007).

The purposes of this study are to investigate the Fenton process expressed in terms of the decolorization, organics removal efficiencies, and their kinetics in order to determine optimal conditions for the Fenton's reagent.

Materials and Methods

Palm oil mill effluent

POME grab samples were collected from the final pond of the stabilization pond wastewater treatment system in a palm oil factory located in Trang province, Thailand. They were placed in a thermal-resistant plastic container and were then refrigerated at a temperature lower than 4°C.

Reagents

Analytical-grade chemicals used during experiments included a H₂O₂ solution (30% w/w), sodium hydroxide (NaOH), and 98% of sulfuric acid (H₂SO₄). Ferrous sulfate hepthahydrate (FeSO₄·7H₂O) was used as a source of Fe²⁺. Deionized water was used throughout the study.

Experimental setup

The experiments were performed in a stirred glass batch reactor containing 250 mL of POME at 26°C±1°C and under atmospheric pressure. POME samples were filtrated through a Whatman grade GF/C glass microfiber filters in order to remove suspended particles before the experiment. The pH of the samples was adjusted by adding H₂SO₄, and NaOH was constantly controlled throughout the experiment.

Oxidation by Fenton reagents

A predetermined amount of catalytic FeSO₄·7H₂O was added to the reactor, and the mixture was vigorously stirred until it dissolved. The Fenton oxidation began with the addition of a H₂O₂ solution. Samples were withdrawn from the reactor at predetermined time intervals. For the analysis of soluble chemical oxygen demand (SCOD), excess amounts of NaOH were added (to adjust pH to 7–10) after 1 h of reaction in order to terminate the Fenton reaction by ferric hydroxo complexes (Fe(OH)₃) precipitation, followed by heating for 1 h and then left overnight at an ambient temperature in order to withdraw the residual H₂O₂ (Kang et al., 1999). The samples were then (I) filtrated through GF/C before analyzing color and SCOD, and (II) filtrated through a GF/F glass microfiber filters before measuring the fluorescence excitation-emission matrix (FEEM).

The Fenton condition used was estimated from pretest experiments and other researches (Meriç et al., 2004).

The photo-Fenton reaction was performed under 6 Watt UV lamp power (UVP model UVGL-58) providing wavelengths ranging from 254 to 365 nm.

Analytical methods

BOD₅, COD, TSS, oil and grease, dissolved organic carbon (DOC), and pH were analyzed according to the Standard Methods 5210B, 5220A, 2540D, 5520B, 5310B, and 4500H, respectively (APHA, 2005). Duplications of all analyses were carried out for each filtered POME sample. The [H₂O₂] was measured with the potassium permanganate method (Mendham et al., 2000). Decolorization of POME was determined by measuring the absorption intensity of the samples at wavelengths of 475 nm (Prasongsuka et al., 2009) using UV-visible spectrophotometry. FEEMs were measured using a spectrofluorometer. The FEEM spectra of deionized water was subtracted from the FEEM spectra of all samples, and converted to quinine sulfate units (QSU). 10 QSU is equivalent to the fluorescence spectra of 10 μg L⁻¹ quinine sulfate solution at 450 nm with an excitation wavelength of 345 nm.

Results and Discussion

POMW and POME characteristics

The characteristics of POMW and POME are presented in Table 1. The POME remained organic with a low BOD₅/COD. The absorbance spectrum of the POME at a wavelength range of 200 to 800 nm is presented in Fig. 1, of which the peak intensity was not evident. This characteristic corresponded to the spectrum of natural water containing organic matters. The UV absorbance at 254 nm (UV-254) value was not available. This indicated that the POME contained high quantities of organic matter, especially of aromatic structures. The FEEM peak intensities of POME were identified at Ex/Em of 315/415 nm (peak A), and 260/445 nm (peak B) as shown in Fig. 2. The appearance of fluorescent peaks A and B indicated respectively the existence of humic and fulvic acid-like substances, respectively (Musikavong et al., 2007), which caused colors in the POME, and not only from tannin and lignin.

FIG. 1.

Absorbance spectra of POME at a wavelength range of 200 to 800 nm. POME, palm oil mill effluent.

FIG. 2.

FEEM patterns of POME. Peak A: humic acid-like substances; Peak B: fulvic acid-like substances. FEEM, fluorescence excitation-emission matrix.

Table 1.

The Characterization of Palm Oil Mill Wastewater and Palm Oil Mill Effluent

Parameter	POMW	POME
Color	Dark brown	Brown
pH	4.2–4.8	6.8–9.1
BOD₅ (mg L⁻¹)	21,500–70,500	12–382
COD (mg L⁻¹)	37,726–166,483	120–638
DOC (mg L⁻¹)	–	121–140
TSS (mg L⁻¹)	5166–50,050	4–278
Oil & grease (mg L⁻¹)	1132–14,346	1.5–14.0

POMW, palm oil mill wastewater; POME, palm oil mill effluent; BOD₅, biochemical oxygen demand; COD, chemical oxygen demand; DOC, dissolved organic carbon; TSS, total suspended solids.

The role of H₂O₂ and Fe²⁺dosage

The effect of [H₂O₂] in the range of 10 to 150 mM was investigated at a pH value of 3±0.2, [Fe²⁺], 1.0 mM, and 5 min reaction time decolorization, and 1 h for SCOD removal. The relationships between the decolorization and SCOD removal efficiencies are shown in Figs. 3 and 4. It was found that the decolorization and SCOD removal of POME depended on the initial [H₂O₂]. The optimum efficiencies for both decolorization and SCOD removal were obtained at 50 mM of [H₂O₂]. This indicated that •OH increased with increasing of initial [H₂O₂] up to a certain limit (Burbano et al., 2005). This is due to the decrease of [•OH] as demonstrated in Equations (4), (5), and (10), respectively. The decolorization was faster than the SCOD removal, because the breakdown of bond cleavage in the chromophore group by the reaction of •OH is rather fast and easier than the transfromation of organic substances to carbondioxide, as Fe²⁺ is a catalytic agent in the Fenton process that generates the •OH. The [H₂O₂] 50 mM with [Fe²⁺] varying from 0.2 to 2.5 mM were studied and shown in Figs. 3 and 4. It is indicated that the color and SCOD removal efficiency increased with initial Fe²⁺ concentrations in the range of 0.2 to 1.0 mM. Above 1 mM of [Fe²⁺], the reaction was inhibited. This is because •OH and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes {10} {9} {7} {6} \begin{document} $${}^{ \bullet}{ \rm HO}_2$$ \end{document} were suppressed by Fe²⁺ and Fe³⁺ with a high k value, resulting in less reaction with organic matter, as demonstrated by Equations (2), (7), and (8) (Wongniramaikul et al., 2007). Therefore, the optimum [H₂O₂] and [Fe²⁺] were, respectively, 50 and 1.0 mM for color and SCOD removal of the POME.

FIG. 3.

Effect of [H₂O₂] and [Fe²⁺] on the decolorization (conditions: pH=3.0±0.2; reaction time 5 min; at 475 nm). H₂O_2, hydrogen peroxide; Fe²⁺, ferrous ions.

FIG. 4.

Effect of [H₂O₂] and [Fe²⁺] on the SCOD removal (conditions: pH=3.0±0.2; reaction time 1 h). SCOD, soluble chemical oxygen demand.

The role of pH

The effects of pH on the POME color removal at various times are presented in Fig. 5. The operation at pH 3±0.2 with 50–75 mM of [H₂O₂] and 1.0 mM of [Fe²⁺] yielded the maximum reaction rate. The color and SCOD removal efficiencies of the POME decreased from 58.5% to 34.2% with an increasing of pH value. This is because increasing in pH value above 3.0 leads to the precipitation of Fe(OH₃), resulting in the inhibition of catalytic oxidation (Neamtu et al., 2003).

FIG. 5.

Effect of pH on the decolorization at various times (conditions: [H₂O₂] 50 mM; [Fe²⁺] 1.0 mM; pH=3.0±0.2; at 475 nm).

The role of UV irradiation

The effect of UV on the POME decolorization is shown in Fig. 6. The color removal efficiencies obtained by using UV at wavelengths of 254 and 365 nm were 81.0 and 80.9%, respectively. Although the UV can stimulate the oxidation reaction according to Equation (11), the organic substances can be oxidized not only by production of Fe²⁺ but also by reactive oxidants of •OH as shown in Equation (12) (Sun et al., 2009). In this study, the use of differing UV wavelengths yielded no significant changes in decolorization. This is because oxidation reaction in the presence of UV wavelength 254 nm generated a small amount of •OH, whereas no •OH was produced by the 365 nm UV.

FIG. 6.

Effect of UV and oxidation processes on the decolorization (conditions: [H₂O₂] 50 mM; [Fe²⁺] 1.0 mM; pH=3±0.2; at 475 nm).

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} { \rm H}_2{ \rm O}_2 + hv \rightarrow 2 \bullet { \rm OH} \tag { 11 } \end{align*} \end{document}

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} { \rm Fe}^{3 + } + { \rm H}_2{ \rm O} + hv \rightarrow \bullet { \rm OH} + { \rm Fe}^{2 + } + {\rm H}^ + \tag{ 12 } \end{align*} \end{document}

Monitoring of color, SCOD, and organic substances by FEEM spectroscopy

Results were shown in Fig. 7. It was found that the efficiencies of humic and fulvic acid-like substances reduction were 93.7% to 99.6%, respectively, which corresponded to the decrease of color in the POME at 475 nm. This is due to the breakdown of bond cleavage of organic pigments by •OH, but it was different from the reduction of SCOD, because most of the humic and fulvic acid-like structures were broken down but only certain organic molecule can be oxidized and released as carbon dioxide (Wu et al., 2010).

FIG. 7.

Removal efficiency of color, SCOD, humic, and fulvic acid-like substances (conditions: [H₂O₂]/[Fe²⁺]=0.5–150; pH=3±0.2; and reaction time 1 h.).

Kinetics studies

The kinetics of the POME decolorization reaction based on the relationship between color intensity and reaction time by using the first oxidation step (Lu et al., 1999) was examined. It was found that the color reduction was a pseudo-first-order reaction. A calculation of k was obtained with a good relationship (R²≥0.8) for all [H₂O₂]/[Fe²⁺] ratios (Fig. 8). The k increased with the increasing of [H₂O₂]/[Fe²⁺] ratios up to a certain limit with a color removal efficiency of 61.11%. The DOC value after treatment decreased with the increase of the treatment period (Fig. 9). The DOC removal efficiencies were 37.0%, 64.0%, and 85.0% at the treatment periods of 15, 60 min, and 24 h, respectively. The DOC removal reaction found was of a pseudo-second-order form. This is due to the remaining organic matter in the POME after oxidation; the organic removal reaction then was slower than the decolorization. The k of DOC removal reaction calculated was 2.19×10⁻⁴ (mg/L)⁻¹ min⁻¹.

FIG. 8.

Decolorization kinetics of POME.

FIG. 9.

DOC decrease of POME (conditions: [H₂O₂] 50 mM; [Fe²⁺] 1.0 mM; pH=3.0±0.2). DOC, dissolved organic carbon.

Conclusions

The optimum Fenton conditions for color and organic matter removal of POME obtained from this study were pH 3±0.2 and [H₂O₂]=50 mM with [Fe²⁺]=1.0 mM. Under these conditions, more than 90% of color removal at 30 min and 82.0% of SCOD removal at 1 h was obtained. The k value of decolorization and DOC removal was 6.20×10⁻² min⁻¹, and 2.19×10⁻⁴ (mg/L)⁻¹ min⁻¹, respectively. The presence of UV showed no effect on decolorization. The existence of humic and fulvic acid-like substances caused colors in the POME, and not only from tannin and lignin. The reduction of FEEM intensity peak was highly related to the color reduction.

Footnotes

Acknowledgments

The authors appreciate combined financial supports from the Faculty of Engineering, Prince of Songkla University (PSU), Thailand (contract reference number ENG-53-2-7-02-0002-S-1), the Green Technology Research Unit, and the PSU Graduate School. In addition, kind suggestions from Assoc. Prof. Dr. Jin Anotai are greatly acknowledged.

Author Disclosure Statement

No competing financial interests exist.

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Decolorization and Organic Removal from Palm Oil Mill Effluent by Fenton's Process

Abstract

Abstract

Introduction

Materials and Methods

Palm oil mill effluent

Reagents

Experimental setup

Oxidation by Fenton reagents

Analytical methods

Results and Discussion

POMW and POME characteristics

The role of H2O2 and Fe2+dosage

The role of pH

The role of UV irradiation

Monitoring of color, SCOD, and organic substances by FEEM spectroscopy

Kinetics studies

Conclusions

Footnotes

Acknowledgments

Author Disclosure Statement

References

The role of H₂O₂ and Fe²⁺dosage