Effect of incorporation of ozone prior to ECF bleaching on pulp, paper and effluent quality

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Contents lists available atScienceDirect

Journal of Environmental Management

journal homepage:www.elsevier.com/locate/jenvman

Research article

E ﬀ ect of incorporation of ozone prior to ECF bleaching on pulp, paper and e ﬄ uent quality

Daljeet Kaur

^a,b

, Nishi K. Bhardwaj

^a,∗

, Rajesh Kumar Lohchab

^b

aAvantha Centre for Industrial Research & Development, Paper Mill Campus, Yamuna Nagar, Haryana, India

bDepartment of Environmental Science & Engineering, Guru Jambheshwar University of Science & Technology, Hisar, Haryana, India

A R T I C L E I N F O

Keywords:

Rice straw Bleaching Ozone Wastewater

Adsorbable organic halides

A B S T R A C T

The pulp and paper industry is highly dependent on forest and water resources. It has more concerns on fair utilization of these resources and their conservation for its further expansion. Present study emphasizes on the use of rice straw (agro waste) in papermaking to protect wood based resources. It further deals with ozone bleaching (Z) prior to elemental chlorine free bleaching that proved to be significant in terms of reducing the effluent load specially the reduction in toxic, recalcitrant and carcinogenic compounds. Z based sequences resulted in pulp brightness of∼85% that was 3.6% higher than the elemental chlorine free bleaching. Bleached pulps of Z based sequences were found to be having better strength properties than elemental chlorine based sequence and thus may be adopted as improved bleaching technology. The analysis of handsheets prepared after pulp bleaching was performed using X-Ray diffraction, ATR-FTIR and SEM. Incorporating ozone stage resulted in marked reduction of 58% and 63% in total solids in bleaching wastewater. Reduction of more than 80% in BOD, COD and adsorbable organic halides was achieved in Z based bleaching in comparison to chlorine bleaching. The amount of chlorophenols, guaiacols, catechols, vanillins and syringols became negligible (approx. 90% reduction) in effluents of Z based bleaching sequences. The chlorine dioxide followed by peroxide bleaching after Z stage was found to be the most promising to reduce the effluent load.

1. Introduction

The pulp and paper industry is one of the fastest growing industries worldwide and it depends on woody raw materials for its expansion.

The demand for paper has amplified in a pace that the accessible forest resources may become insufficient to fulfill this requirement (Mohieldin, 2014). Increase in global environmental issues like defor- estation, global warming, climate change and pollution of natural ecosystems further accelerated the need for environmental awareness to protect and manage the environmental resources. The paper industry is now focusing on exploring the potential of non-woods (agricultural residues and annual plants) to produce pulp and paper of promising quality (Sridach, 2010). Rice straw is the most abundant lignocellulosic agro residue available in wood short countries like India and China and can be used as a low cost substitute for woody raw materials. Other- wise, outsized paddy straw is burnt infields causing severe road acci- dents and health hazards (Kaur et al., 2018b).

Soda-AQ (anthraquinone) pulping is the most preferable process for converting the non-wood raw materials to ﬁbrous pulp (Yue et al., 2016). The chromophoric groups of lignin in brown colour pulp are

further removed using different bleaching agents. The pulp and paper industry is allied with huge consumption of fresh water and generation of effluents loaded with toxic components (Singh and Dutt, 2014). Due to discharge of bleaching effluents by paper mills into nearby water bodies, which causes serious problems to aquatic plants and animals, Ministry of Environment and Forests in India has put this sector in the Red Category list of 17 industries (Kumar et al., 2015). Bleaching effluents are loaded with biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended solids (mainlyfibers), fatty acids, tannins, resin acids, lignin and its derivatives. The severity of toxicity depends on type of raw materials and bleaching chemicals used for papermaking (Covinich et al., 2014). Bleaching chemicals, especially the chlorine and its derivatives, participate in various oxidation-sub- stitution reactions during bleaching and result in formation of biorefractory organochlorine compounds (Kaur et al., 2017a,2017b;2018a, 2018b). There are about 500 different chloroorganic compounds found in effluents of elemental chlorine based bleaching sequences namely chlorophenols (CP), chlorocatechols (CC), chloroguaiacols (CG), chlorosyringols (CS), chlorosyringaldehydes (CSA), chlorovanillins (CV), chlorinated resin and fatty acids (cRFA), chlorinated

https://doi.org/10.1016/j.jenvman.2019.01.089

Received 20 November 2018; Received in revised form 14 January 2019; Accepted 20 January 2019

∗Corresponding author.

E-mail addresses:[email protected](D. Kaur),[email protected](N.K. Bhardwaj),[email protected](R.K. Lohchab).

T

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hydrocarbons etc. (Freire et al., 2003). Chloroorganic compounds are severely known for their toxicity as carcinogenic, adsorbed by the skin, paralytic hits, nausea, lungs malfunctioning, cardiac diseases and in- jurious impacts on reproductive, developmental and hematological organs of diﬀerent organisms (Waye et al., 2014;Kumar et al., 2005).

Chlorinated compounds not only elevate the BOD and COD of eﬄuents but also impart acute and chronic toxicity in it (Veeramani, 2005).

These biorefractory compounds pass on to the different trophic levels with higher magnification of toxicity and high BOD, COD of effluent menace the aquatic life due to eutrophication and enhanced growth of algal blooms in water bodies (Gamal et al., 2005; Mengesha et al., 2004). Discharge of bleaching effluents not only disturbs the integrity of water resources and environment but also brings our anxiety in the direction of human health and reduction of productivity of our natural ecosystems as a result, presenting both economic and social risks. These reasons have put immense pressure on pulp mills to reduce the expul- sion of effluent loaded with BOD, COD, total suspended solids, AOX and colour through new wastewater treatment techniques (Teh et al., 2016).

The pulp and paper industry is setting its goals on technical in-situ process modifications to reduce wastewater discharge by addition of oxygen delignification, extended cooking, replacement of chlorine with chlorine dioxide (ClO2) and use of strong oxidizing agents like hydrogen peroxide and ozone (Meenakshi et al., 2011). Ozone is the most powerful bleaching agent and it reacts with the pulp just in few minutes in comparison to long time taken by chlorine and other bleaching chemicals. In mills, ozone is generated from oxygen in an ozone generator having a number of electrodes and used instantly after produc- tion (Bajpai, 2015). The excess of generated ozone is dissociated back to oxygen so this process is eco-efficient as there is no discharge of toxic chemical. Ozone bleaching (Z stage) was started about 20 years back

with a view of reducting toxicity of bleaching wastewater using total chlorine free (TCF) bleaching methods (Metais et al., 2011). World- wide, there are approximately 22 mills using Z stage followed by chlorine dioxide and hydrogen peroxide bleaching for different raw materials like hardwoods, softwood and blends of both in different proportions under light elemental chlorine free (ECF) bleaching (Metais et al., 2011). Z stage has been reported as well competent and viable in terms of lignin removal, price tag and environmental security (Shatalov and Pereira, 2008). Addition of Z stage prior to ECF bleaching possesses a great potential to reduce the amount of effluent generation, COD, BOD and colour in comparison to direct ECF bleaching (Wennerstrom et al., 2007).Liebergott (1982)also found the drastic reduction in COD and colour in effluents of Z stage in comparison to oxygen delignification. Incorporation of oxygen and Z stage reduces the adsorbable organic halide (AOX) content by 98% from bleach plant effluents (NCASI, 2009). Ozone in earlier few seconds of reaction attacks the lignin chains and ensues with destruction of long chainfibers leading to loss offiber strength (Jablonsky, 2009). Therefore, the optimization of Z stage is crucial before the treatment to protect the pulp and effluent quality (Liebergott, 1982). Many studies have been conducted so far using ozone as a stage during bleaching of hardwoods and softwood (Wennerstrom et al., 2007;Liebergott, 1982;Jablonsky, 2009) but no literature is available for rice straw till date.

The article aims at reducing the pollution load of the bleaching eﬄuents on incorporating the ozone stage prior to elemental chlorine free bleaching of rice straw pulp. The study signiﬁes the importance of using rice straw to eliminate the problem of its management and burning which may reduce the environmental problems.

Fig. 1.Schematic representation of experimental work.

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2. Experimental

Present research was conducted using∼15 kappa number soda-AQ rice straw pulp for bleaching studies. The pulp wasﬁrst bleached with conventional bleaching sequences to prepare the pulp and eﬄuents.

The reduction in chlorophenolic compounds of diﬀerent sequences was studied. In a sequence (ZDP), ozone (Z stage) was incorporated at initial stage proceeded by ClO2 stage (D) and replacing the ClO2(D) with hydrogen peroxide (P) at last stage for further reducing the amount of BOD, COD, colour, AOX and generation of chlorophenolic compounds.

The pulp properties of diﬀerent sequences were also analyzed. The results were presented in terms of pulp and wastewater quality of different bleaching sequences. The schematic representation of the experimental work has been presented inFig. 1.

2.1. Soda-anthraquinone (AQ) pulping of rice straw

Rice straw was collected, washed and chopped into small pieces of 10–15 mm size. It was converted toﬁbrous pulp using the soda-AQ pulping method in an autoclave batch digester (6 batch having 150 g of rice straw in each). Soda pulping method is a chief practice as far as agro residues are concerned (Kaur et al., 2017a,2017b; 2018a;2018b).

This process involves the oxidative degradation of lignins which sta- bilizes the carbohydrates and results into elevated yields. Rice straw was mixed with soda liquor (12% on oven dry basis of raw material) and water in a bath ratio of 1:4 then cooked for 20 min at 155 °C. For accelerating the deligniﬁcation rate, 0.05% of AQ was added during pulping. The black liquor was separated from the pulp after pulping.

The pulp was then disintegrated and extensively washed withﬁltered water to remove all residues of black liquor. The pulp was screened in a Somerville screen to separate out the uncooked material and shredded for homogenization. The pulp produced was of 37.3% ISO (where ISO stands for International Standards Organization) brightness. It ex- hibited strength properties like viscosity, tensile, tear and burst index as 14.2 cP, 42.2 Nm/g, 2.12 mN m²/g and 2.46 kN/g, respectively.

2.2. Ozone stage (Z) prior to ECF

The soda-AQ pulp was acidified (pH 2.0) with 4N H2SO4 before being processed for ozone stage bleaching. Afterwards, pulp was cen- trifuged at 5000 rpm for about 30 min to drain out the water and acid for maintaining the consistency of 27%. The 50 g oven dry pulp was divided in three equal parts and each was thenfluffed in a mixer for 10 s tofibrillate thefibers so that ozone can react with eachfiber of the pulp. Pulp was warmed in microwave for 30 s to attain the temperature of 40 °C. Ozone was generated using oxygen in an ozone generator (Make- ORAIPL, Model-INDOZ-30) by electric arc method and si- multaneously transferred to the ozone reactor containing pulp. The strength of generated ozone was calculated before using it in bleaching reaction by absorbing it in 2% solution of KI buffer which was further titrated with 0.1N sodium thiosulphate using starch as an indicator. For optimizing the dose, aflow of 1.5 lpm of ozone was applied to 50 g oven dry pulp for reaction time of 1, 2, 3, 5 and 7 min which corresponds to 1.53, 3.16, 4.74, 8.0 and 11.2 kg/t of ozone dose. After completion of the reaction, pulp was transferred to polythene bag and processed for its extraction stage (E). For E stage, pulp was well mixed with sodium hydroxide and water to maintain the pH 11.0 and consistency of 10%.

The pulp was kneaded and kept in a water bath at a temperature of 75 °C for 30 min. The optimization process was repeated twice. It was then washed properly and used for analyzing the kappa number using a standard TAPPI (Technical Association of Pulp and Paper Industry) test method T 236 om-99 that depicts the residual lignin content within the pulp. The analysis of variance (ANOVA) for kappa number was calculated using software origin 6.1. The pulp yield was measured by following the method used by (Young, 1996). TAPPI test method T 230 om-99 was used for determining the pulp viscosity which indicates the

degradation of long chainﬁbers due to attack of bleaching chemicals.

The brightness of the pulp achieved in ozone stage at diﬀerent dose was also analyzed using a L & W Elrepho brightness tester. On the basis of these pulp properties, the ozone dose was selected for bulk experiments and processed for further bleaching stages like chlorine dioxide and peroxide.

2.3. Bleaching of rice straw pulp

The unbleached pulp was ﬁrst bleached with conventional bleaching sequence CEOPHH where C denotes the chlorine gas, EOP

represents the extraction stage using sodium hydroxide, oxygen and hydrogen peroxide and H is for calcium hypochlorite stage. Keeping the wastewater quality in consideration the pulp was also bleached with sequence DEOPD where D denotes the ClO2. Chlorine water was prepared in lab by solublizing the chlorine gas in water having temperature 10 °C. Other commercial grade bleaching chemicals were procured from a nearby paper mill. In the CEOPHH sequence, a chlorine dose of 4.2%

was applied to pulp maintaining 3% consistency for 45 min at 40 °C in C-stage followed by 1.0% of calcium hypochlorite at H1stage and 0.5%

was at H2stage for 120 min at 45 °C. During DEOPD sequence, 3.8% of chlorine dose was applied to the pulp for 45 min at 55 °C in initial D stage followed by 0.9% of chlorine dioxide for 180 min at 75 °C in D1

stage. During EOPstage in both the sequences, the pulp was bleached by applying the 1.8% of NaOH, 0.5% of hydrogen peroxide and 5 kg/cm² of pressure at 80 °C for 120 min. The consistency of 5% was maintained in stage D and 10% in stages H, D1and EOP.The experimental conditions for bleaching are given inTable 1.

Further, The Z pulp was bleached with sequences DEOPD and DP.

The dose of ClO2and peroxide was optimized for both the sequences.

Toﬁnd the chlorine demand after Z, the kappa factors 0.20, 0.22, 0.24, 0.26 and 0.28 were applied. In D1stage of DEOPD sequence, dose of 0.5%, 0.7%, 1.0%, 1.3%, 1.6%, 1.9% was applied. In ZDP sequence also dose in D stage was optimized by taking the dose range of 1.0%, 1.5%, 2.0%, 2.5%, 3.0% and 3.5% of ClO2. Likewise, the dose of hydrogen peroxide was also optimized by applying the dose 0.1%, 0.25%, 0.5%

and 0.75%. The optimized value for all the chemicals was selected on the basis of residual content in bleaching effluents and brightness achieved in pulp. The experiments for bulk bleaching treatments (CEOPHH, DEOPD, ZDEOPD, ZDP) were conducted on a selected dose of different bleaching agents. The pulp was washed properly after bleaching and used for its characterization. All the bleaching sequences were conducted in duplicates. The effluent generated after each stage was also collected and characterized for its environmental quality.

2.4. Analysis of bleached pulp properties

The diﬀerent pulp properties were measured using the standard methodology of Indian Standards (IS) and technical Association of Pulp Table 1

Experimental conditions for bleaching.

Parameters Z C D EOP H H D1 P

Chlorine applied (%) – 4.2 – – – –

Chlorine dioxide applied (%)

DEOPD – 3.8 0.9 –

ZDEOPD 2.6 1.6 –

ZDP – 2.5 –

Ozone (kg/t) 1.53 –

Hypo applied (%) – – – – 1.0 0.5 – –

NaOH (%) 1.5 – – 1.8 0.2 0.1 – 1.8

Peroxide (%) – – – 0.5 – – – 0.75

Consistency (%) 27 3.0 5.0 10 10

Initial pH 11.5 1.9 5.5 12.0 11.0 10.0 4.0 11.5

End pH 10.7 2.0 2.9 10.9 9.0 8.5 3.5 10.5

Temperature (°C) 40 40 55 80 45 75 75

Time (min) 1 45 45 120 45 180 120

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and Paper Industry (TAPPI). The moisture content of the pulp was analyzed using method IS 1060 (Part I)-1966. The micro kappa number of the pulp after EOPstage was analyzed using method UM-246. Washed bleached pulps were further used for measuring the viscosity using TAPPI test method T 230 om-99 and preparation of handsheets (10 for each bleaching sequence) of 60 g/m² (T 205 sp-97). The Schopper- Riegler number was also analyzed by using test method ISO-5267. The handsheets were used for analyzing the strength properties like tear index with L & W tearing tester, SE 009, burst index with L & W bursting strength tester, code 181 and tensile index with L & W tensile strength tester, code 060 following tappi test methods T 414 om-88, T 403 om-91 and T 494 om-01, respectively. The optical properties of pulp like brightness, whiteness and opacity were also measured using L

& W Elrepho brightness tester, code 070/071. For calculating the uncertainty range, 6 values for all optical and strength properties were taken into consideration.

2.5. X-ray diﬀraction (XRD)

Handsheets prepared from bleached pulps were analyzed for calculating the crystallinity index of cellulose by using MiniFlex II desktop X-ray diffractometer. Intensity of the diffraction peaks for different sheets was recorded from 10⁰to 60⁰of scattering angle (2θ) at 0.05⁰/

scan of scanning speed. XRD pattern of diﬀerent handsheets was used for determining their respective cellulose crystallinity index (Xc) using the Equation(1)(Kaur et al., 2016):

= ×

Xc I Iamp

% ( 002 –I )

002 100

(1) whereI002is the respective peak intensity of crystalline phase andIamp

is the peak intensity of amorphous phase from the (002) lattice plane.

2.6. ATR-FTIR analysis

Frontier MIR LiTa/KBr/AI spectrometer (PerkinElmer, UK), was used for obtaining the ATR-FTIR spectra of handsheets prepared from the bleached pulp of the sequences studied. The refractive index of the crystal (diamond) used in the ATR cell (from PIKE Technologies) was 2.4. Incident radiation at an angle of 45° was applied to the samples.

The spectrum of the samples was obtained using 32 scans, in the range of 4000 to 400 cm⁻¹wavelength, at a resolution of 8 cm⁻¹to elucidate the changes.

2.7. SEM analysis of handsheets after bleaching

Tofind the effect of different bleaching sequences on fiber morphology, handsheets of bleached pulp were cut into the size of 1 × 1 cm and coated with goldfilm of thickness 5 nm. The samples were analyzed with scanning electron microscope (JEOL JSM-6510 LV) and micrographs at different resolution were taken. In present study, the scanned micrographs at 2500× magnification were used to compare the dif- ferences.

2.8. Eﬄuent characterization

The collected effluents after each stage of bleaching sequence were mixed in their respective volumetric proportion to make the composite samples. The effluent from each stage and composite effluents of different bleaching sequences were characterized with standard methods of IS and APHA (24th edition) in terms of BOD3(IS:3025 Part 38 and 44), COD (IS:3025 Part 58), TOC (APHA 5310 D), pH (IS:3025 Part 11), lignin (22nd edition APHA 5550), colour (22nd edition APHA 2120 C), AOX (ISO 9562), TS (IS:3025 Part 15), TDS (IS:3025 Part 16) and TSS (IS:3025 Part 17). The 10 values for COD, BOD, TS, TSS, TDS and colour were taken into account for calculating the uncertainty range for these parameters. But for AOX, the analysis was performed thrice.

The composite effluents of sequences CEOPHH, DEOPD, ZDEOPD and ZDP were analyzed for chlorophenolic compounds using gas chromatography. Samples were extracted and derivatized according to the method suggested byLindström and Nordin (1976). The experiments were conducted in triplicates. During extraction, 500 mL of composite effluent sample of each sequence was taken and the pH was maintained to∼2 using 1 N sulphuric acid. A solvent mixture of HPLC grade diethyl ether and acetone (90:10) was prepared and 200 mL of this solvent mixture was added to effluent in a separating funnel for extraction with intermittent shaking. After 48 h, whole ethereal extract was separated in another separating funnel and emulsion formed was broken by heat gun. To make the ethereal layer free from acids, it was washed with 2.5 mL of 0.5 M sodium bicarbonate. It was further shaken with 2.5 mL of 0.5 M sodium hydroxide; the aqueous NaOH layer with chlorophenols was further washed with fresh diethyl ether. The aqueous NaOH layer was then derivatized to acetyl chlorophenols with 0.5 mL of acetic anhydride. Acetyl derivatives werefinally extracted in 4 mL of n- hexane (HPLC grade) and analyzed using gas chromatography with ECD detector. About 34 isomers of various chlorophenolic compounds supplied by Sigma Aldrich, USA were used as reference compounds.

Various chlorophenols were detected by matching their retention time ( ± 0.5 min) with those of pure standards.

2.8.1. GC conditions

GC of make Varian-450 equipped with ECD detector was used for the qualitative and quantitative analysis of chlorophenolic compounds.

The speciﬁcations of column used for separation are given below. Three gases nitrogen as a carrier gas, hydrogen as a fuel gas and oxygen as an oxidizer gas were used in the process. ECD used radioactive foil of⁶³Ni (15 mCi) that emits ß particles (low energy electrons) which when collides to the nitrogen gas are converted to high energy electrons es- tablishing high standing current. The sample carried by make up gas to the detector, electron absorbing analyte absorbs the electrons and current is reduced between the cathode and collector anode. The change in current was recorded in the chromatogram.

Column type Factor four Capillary column (VF-1ms) Column dimensions 30 m × 0.25 mm I.D. with 0.25μm thickness

Detector type ECD

Sample size (μl) 1

Detector temperature (°C) 290

Injector 70

Column temperature (°C) Initial 100 for 3 min 100–180 @ 4 °C min⁻¹ 180 for 10 min 180–270 @ 15 °C min⁻¹ 270 for 2 min

Column pneumatics 1 mL min⁻¹

Make up nitrogenﬂow 28 mL min⁻¹

Split ratio 1:20

2.8.2. Extraction eﬃciency

Extraction and derivatization for standard solutions was performed by same method as used for samples for calculating the extraction ef- ﬁciency of each chlorophenolic compound and their peak area was identiﬁed using GC. The quantity of chlorophenols in extracts was es- timated on the basis of peak area by using Equation(2):

= Peak area of extracted sample Peak area of non extracted sample

Extraction efficiency (%) *100

(2) Results were depicted in terms of reduction in eﬄuent load of different environmental parameters specially the chlorophenolic compounds in diﬀerent bleaching sequences.

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3. Results and discussions

3.1. Optimization of bleaching chemicals for ZDEOPD and ZDP sequences 3.1.1. Optimization of ozone dose

During optimization of Z stage, Ozone dose of 1.53, 3.16, 4.74, 8.0 and 11.2 kg/t was applied to rice straw pulp. It was found that on increasing the dose from 1.53 to 4.74 kg/t, the residual ozone content in the bleaching effluent increased from 0.12 to 0.14 kg/t showing that more than 90% of the ozone was utilized for bleaching the pulp. On further increasing the ozone dose, residual ozone content highly increased in effluent leading to wastage of ozone generated with high effluent load. On increasing the dose of ozone, delignification rate also got enhanced and the kappa number of the pulp was reduced from 15.2 to 9.8 at 1.53 kg/t dose and 7.5 at 3.16 kg/t. The same observation of reduction in kappa number of softwood kraft pulp was also reported by (Rudenius, 2014). This may be attributed to the fact that ozone as a strong oxidizing agent reacts more with muconic acid derivatives of the pulp and also cleave the C]C bond more efficiently than other bleaching agents which contributes to higher delignification and low- ering in kappa number (Lachenal et al., 2006). Afterwards, meagre reduction in kappa number was observed at 4.74 (7.2), 8.0 (7.0) and 11.2 kg/t (6.7) of ozone dose. The variance for kappa number is given inTable 2. The optical properties of the pulp were also studied and it was found that the brightness and whiteness of the pulp enhanced when the dose range of 1.53, 3.16, 4.74 and 8.0 kg/t was applied but at high dose of 11.2 kg/t, the brightness ceiling was observed, may be due to no more possibility of lignin removal at this stage. The viscosity of the pulp was measured which indirectly depicted the level offiber degradation due to chemicals. It was observed that ozone exerted a marked effect on the fibers strength and significant drop in viscosity was found on bleaching the pulp with ozone. The viscosity reduced by 50% on just applying 1.53 kg/t of ozone and reduced further on increasing the dose.

Zhang et al. (2000)in their study on ozone bleaching also observed that ozone directly attacked the glycosidic bonds of the glucose units leading to cellulose degradation. Many studies also revealed that the loss of viscosity during ozone bleaching was due to formation of hydroxal and hydroperoxy radicals during its reaction with lignin (Ragnar et al., 1997;Pouyet et al., 2013). The lignin radicals are less selective than ozone itself and correspond to more cellulose degradation. Study by Tripathi et al. (2018a) revealed that the bleaching process variables played an important role in deligniﬁcation during ozone bleaching;

more eﬀective and selective bleaching occurred at lower pH. But many studies have proved that the reaction kinetics of ozone with lignin is more than kinetics for carbohydrates oxidation thus as long as lignin remains in the pulp, a well operated bleaching system will not degrade the cellulose ﬁbers. This statement was not found to be true during ozone bleaching of rice straw which may be due to less lignin content in agro residues compared to hardwoods and softwoods. Tripathi et al.

(2018a)also reported that viscosity of the ozone-treated wheat straw pulp was improved with decrease of ozone dose, increase in pulp

consistency, and reduction of reaction pH below 4. The eﬀect of increasing dose on pulp brightness and viscosity is depicted inFig. 2.

On the basis ofﬁnding sharp decline in viscosity on increasing the ozone dose, 1.53 kg/t of ozone was found to be the optimum and pulp with kappa number 9.8 was used for further bleaching studies.

3.1.2. Optimization of ClO2dose

The optimization of dose for bleaching chemicals for sequences CEOPHH and DEOPD was already done in a previous study byKaur et al.

(2018b). Toﬁnd the chlorine demand after Z stage the ClO2dose was optimized at the initial stage of bleaching. The chlorine demand was calculated using Equation(3)(Hise, 1996) given below:

= ×

Chlorine demand (%) Kappa number Kappa factor (3) On increasing the kappa factor from 0.20 to 0.24, the residual content in the bleaching effluent was very low depicting the more need for chemicals in this stage. The residual ClO2in effluent at 0.24 kappa factor was 25 ppm and increased to 52 ppm at kappa factor 0.26. On increasing the kappa factor above 0.26, the residual ClO2 increased sharply in the bleaching effluent (79 ppm at 0.28 kappa factor) resulting into loss of chemicals as well as high pollution load. So, 0.26 kappa factor denoting the 2.5% of active chlorine applied on the pulp was found to be optimum at this stage.

At the last D stage of the DEOPD sequence, 0.4%, 0.7%, 1.0%, 1.3%, 1.6% and 1.9% of ClO2was applied to the pulp on oven dry basis and it was found that up to 1.0% dose the residual ClO2was near nil in the bleaching eﬄuent, even at 1.3% of dose meagre residual was found.

The residual content was found to be 35 ppm at a dose of 1.6% dose and increased to 48 ppm at 1.9%. On studying the brightness, it was found that the brightness increased with increase in chlorine dose up to 1.6%

after which no eﬀect on brightness was observed as shown inFig. 3. In ZDP sequence also dose of ClO2was optimized and on the basis of residual content and brightness dose of 2.5% was found to be optimum as shown inFig. 3.

In ZDP sequence, during optimization of P stage it was found that 0.75% dose of hydrogen peroxide was enough for achieving the brightness 85.5% ISO with minimal discharge of bleaching chemical in the eﬄuent.

3.2. Eﬀect of bleaching sequences on pulp and paper properties 3.2.1. Eﬀect on optical properties

Incorporation of Z stage prior to ECF bleaching has positive impact on the optical properties of the pulp. In both the Z based sequences ZDEOPD and ZDP a brightness of∼85% was achieved which was higher than the conventional bleaching processes CEOPHH and DEOPD. A study Table 2

One-way ANOVA for kappa number.

Ozone dose (kg/t) Kappa number (mean) Variance

1.53 9.81 0.01667

3.16 7.52 0.0425

4.74 7.20 0.02667

8.0 7.00 0.06

11.2 6.67 0.01583

F = 172.67784 p = 2.38332E-12 At the 0.05 level.

The means of kappa number at different dose are significantly different.

Fig. 2.Eﬀect of ozone on viscosity and brightness of pulp.

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byLiebergott (1982)revealed that on bleaching the 14.5 kappa number kraft hardwood pulp with ZDED sequence brightness of 90% was achieved which was far higher than the brightness achieved for rice straw in present study. Further it was found that elemental chlorine and calcium hypochlorite were more eﬀective to bleach rice straw pulp rather than chlorine dioxide (Kaur et al., 2018b) but incorporation of Z prior to DEOPD improved the eﬃciency of this sequence. The result was found to be in agreement with the studies ofMarlin et al. (2013)and Vilve et al. (2008)on hardwood and softwood. A brightness of 89.7%

was also achieved byLachenal et al. (2006)on implementing the ozone stage prior to DEDD bleaching. The whiteness of the pulp was also improved in ZDEOPD (80.4) and ZDP (78.2) sequences. The yellowness of the pulp was reduced in ZDEOPD and ZDP sequence in comparison to CEOPHH and DEOPD. The diﬀerent optical properties of the pulp after bleaching are mentioned inTable 3.

3.2.2. Eﬀect on strength properties

On bleaching the pulp with diﬀerent sequences, DEOPD sequence was found to be the best as far as strength properties are concerned.

Karim et al. (2011)reported that ClO2has higher selectivity for oxidizing lignin and preserves the pulp quality. Bleached pulp of CEOPHH sequence was of the lowest quality.Kaur et al. (2018b)in their study on bleaching the rice straw soda-AQ pulp found that elemental chlorine reduced the pulp viscosity and other physical properties of the pulp.

Incorporation of Z stage prior to ECF bleaching has little negative impact on diﬀerent strength properties like tear, tensile and burst index when compared to DEOPD alone. The values for diﬀerent strength properties are shown inTable 4. Ozone is a strong oxidizing agent and

takes just few min for the reaction resulting into attack on long chain ﬁbers just after deligniﬁcation. This was indicated by a drop of 27% in viscosity in ZDEOPD and 25% in ZDP sequences in comparison to DEOPD.Rudenius (2014)also found the decline in viscosity of softwood pulp with increase in ozone dose. At higher ozone charge degree of polymerization decreased as studied byTripathi et al. (2018b)on their study on wheat straw.

The tear index of the sequence ZDEOPD and ZDP was found to be less than DEOPD again due to degradation of carbohydrates with attack of ozone and other bleaching chemicals. These results were found in close proximity to the results reported byLiebergott (1982)in his study on ozone bleaching of softwoods. Due to attack of ozone and other bleaching chemicals thefibrillation offibres enhanced, resulting into generation of morefines. It further resulted in an increase in tensile and burst index of the pulp in ZDEOPD and ZDP sequences. Shackford (2003)observed the 10% increase in burst index of hardwood kraft pulp after ZD sequence. The generatedfines absorbed more water and enhanced the Schopper-Riegler number of the pulp in ZDEOPD and ZDP sequence. On comparing the sequences ZDEOPD and ZDP, it was observed that viscosity, tear index and burst index of ZDP sequence were better, that may be due to the just two bleaching stages after Z stage as well as incorporation of peroxide instead of chlorine dioxide in last stage. Rudenius (2014) also found the better delignification by incorporating the peroxide during softwood bleaching. Peroxide stage in bleaching enhanced the pulp properties with better environmental safety (Mustajoki et al., 2010;Tutus, 2004;Shirkolaee, 2009). The use of oxygen in EOPstage and three stages bleaching after Z in ZDEOPD may be responsible for lower strength properties. All pulp treated using Fig. 3.Optimization of ClO2dose at D1stage in ZDEOPD and ZDP sequences.

Table 3

Optical properties of pulps of diﬀerent bleaching sequences.

L* represents the diﬀerent co-ordinates that provide the ideas for shade of the paper.

-ve a* values represent the greenish colour in the pulp.

+ve b* values represents the yellowness of the pulp.

ISO stands for International Standards Organization.

UR represents the uncertainty range of optical properties at a conﬁdence level of 95%.

Parameters CEOPHH UR DEOPD UR ZDEOPD UR ZDP UR

Brightness (% ISO) 83.5 ± 4.1 82.2 ± 4.1 85.5 ± 4.2 85.7 ± 4.2

L* 94.6 93.5 95.7 95.6

a* −0.24 −0.37 −0.12 −0.08

b* 3.0 3.3 1.9 2.2

Whiteness 74.9 ± 3.7 74.1 ± 3.7 80.4 ± 4.0 78.2 ± 3.9

Yellowness 5.7 6.0 3.7 4.4

Where:

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the three ECF bleaching sequences were found to be having better strength properties than CEOPHH. Therefore, this process may be adopted as improved bleaching technology by the paper industry to cut down the elemental chlorine consumption. Metais et al. (2011)also recommended the use of a light-ECF bleaching sequence that provides the same strengths and better optical properties in comparison to ECF alone.

3.3. X-ray diﬀraction analysis

The XRD pattern obtained for all handsheets showed the peak intensity of lattice plane 002 at 2θin the range of 22.2⁰ to 22.6⁰ that denotes the crystalline cellulose (Kaur et al., 2016). The peak intensity for amorphous phase was found at 2θ= 18.7⁰to 19.2⁰. The peak intensity of crystalline cellulose for handsheets of CEOPHH bleaching sequence was at 2θ= 22.42⁰and for amorphous phase, the peak intensity was at 2θ= 18.70⁰. In DEOPD, ZDEOPD and ZDP bleaching sequences, the peak intensity of lattice plane 002 was found at 2θ= 22.58⁰, 22.21⁰ and 22.29⁰, respectively whereas for amorphous phase it was observed at 19.21⁰, 18.69⁰and 18.8⁰respectively as shown in (Fig. 4). The cellulose crystallinity index as calculated by XRD pattern was found to be 70.1% for CEOPHH sequence. The cellulose crystallinity index was observed to be the highest for DEOPD sequence i.e. 74.5%. For ZDEOPD and ZDP sequences, the cellulose crystallinity index was 72.3% and 73.4%, respectively. The highest crystallinity index for DEOPD may be attributed to the selectivity of chlorine dioxide for lignin removal and protection of cellulosic/hemicellulosic fibers during bleaching. In CEOPHH sequence use of elemental chlorine and calcium hypochlorite distort the carbohydrates resulting into lower crystallinity index in comparison to chlorine dioxide and ozone based sequences. The results also correlate to the poor viscosity and tear index of handsheets of CEOPHH sequence pulp as shown inTable 3. The crystallinity index of handsheets ZDEOPD and ZDP sequences was also lower than that of DEOPD. It may be due to the strong oxidizing effect of ozone on long chainfibers that degrade them.

3.4. FTIR-ATR analysis

The structural changes in the rice straw pulp after DEOPD, ZDEOPD and ZDP were studied by ATR-FTIR spectra as shown inFig. 5. FTIR spectra of handsheets made from bleached pulps showed a strong band in range of 3445–3250 cm⁻¹ that represents the OH bond stretching vibrations of carbohydrates and lignin bonding. The band at 3428 cm⁻¹, 3340 cm⁻¹and 3272 cm⁻¹are related to the O(2)H⋯O(6) intramolecular H-bond, O(3)H⋯O(5) intramolecular H-bond, and O(6) H⋯O(3) intermolecular H-bond, respectively, while bands at 3305 cm⁻¹and 3405 cm⁻¹are associated with intermolecular H-bond in 101 plane (Kaur et al., 2016;Fengel, 1993). A shoulder at 2892 cm⁻¹ was observed in absorption spectrum of all the bleaching sequences which showed theeCH asymmetrical stretching vibration in CH3, CH2

and CH groups (Bhardwaj et al., 2017; Kaur et al., 2017). A peak at 1648 cm⁻¹denotes the aromatic vibrations in lignin (Bhardwaj et al., 2006). An absorption band at 1429 cm⁻¹was found that depicted the OCH3 in-plane bending vibrations of syringyl and guaiacyl units in lignin (Zhang et al., 2018). A sharp decrease in these vibrations was observed in ZDEOPD and ZDP bleaching sequences that might be attributed to removal of syringols and guaiacols in these sequences. The bands at 1243-1317 cm⁻¹were observed that denoted the skeletal vibrations of lignin (Reis et al., 2017). The absorption of the band at 3321 cm⁻¹ decreased in Z based sequences representing the low amount of phenolic structures in these treatments. Peaks at 1140 cm⁻¹ and 1029 cm⁻¹ can be assigned to the stretching mode of CeOeC linkages. The peak at 890 cm⁻¹ was increased in ZDEOPD and ZDP sequences that showed the cleavage of the ß-glycosidic bonds and degradation of some carbohydrates during these sequences (Zhang et al., 2018).

3.5. SEM analysis

Scanning electron micrographs of diﬀerent bleached pulp samples Table 4

Strength properties of bleached pulps.

Tear index (mNm²/g) 4.44 ± 0.54 5.421 ± 0.57 5.12 ± 0.56 5.20 ± 0.56

Tensile index (Nm/g) 44.4 ± 2.9 46.9 ± 3.1 47.2 ± 3.0 47 ± 2.9

Burst index (kN/g) 2.87 ± 0.16 3.34 ± 0.18 3.24 ± 0.17 3.27 ± 0.17

Double Fold 24 76 85 87

Viscosity (cP) 5.7 8.4 6.1 6.3

°SR 40 46 48 47

Where UR represents the uncertainty range of paper parameters at a conﬁdence level of 95%.

Fig. 4.X-Ray diﬀraction of handsheets of bleached pulps.

Fig. 5.ATR-FTIR spectra of handsheets of bleached pulps.

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represented the irregular surface morphology due to fiber matrix as shown in Fig. 6. Pulping and bleaching are the processes associated with delignification and also affect the carbohydrate structure.

Bleaching chemicals impose the strong impact on thefinalfiber structure and morphology. The SEM analysis (image-A inFig. 6) showed the highest deterioration in chlorine based sequence i.e. CEOPHH (viscosity 5.7) whereas the chlorine dioxide has high selectivity for removing the lignin and protect the carbohydrates during bleaching process. The similar results were observed in SEM (image B ofFig. 6) representing the DEOPD sequences. In ozone based sequences the distortion of cellulose was less than chlorine based sequence. In both ozone based sequences the viscosity was almost similar therefore the fiber structure can be assumed to be similar as depicted in SEM images C and D.

3.6. Eﬀect on eﬄuent characteristics

On bleaching the pulp with CEOPHH sequence 59.3 m³, in DEOPD and ZDEOPD 37 m³and in ZDP 18 m³of effluent was generated. The CEOPHH sequence was found to be the most polluted followed by DEOPD. Incorporation of Z stage prior to ECF bleaching was found to be a better choice as all the environmental parameters got reduced in ZDEOPD and ZDP sequences. On analyzing the effluent characteristics at each stage, it was found that the extraction stage effluent was most polluted as the chlorolignin compounds separated during the first bleaching stage were extracted by adding the alkali and imparted more COD, BOD, colour and lignin to this stage effluent. The studies by Pokhrel and Viraraghavan (2004)andPanwar et al. (2004)also showed the high amount of environmental parameters in bleaching effluent of extraction stage. The first stage bleaching effluent was also highly contaminated with different environmental parameters as most of the chemical charge required for delignification during bleaching was applied at this stage. The bleaching stages after EOPwere less polluting as

−12% of lignin in comparison to the ozone bleaching was left for separation at these stages that required low chemical charge for de- ligniﬁcation and reduced the eﬄuent load as shown inFig. 7.

A marked reduction of 58% and 63% was found in total solids in effluent of ZDEOPD and ZDP sequences. The effluent characteristics of different sequences are presented inTable 5. The total dissolved solids were also reduced to 29% in DEOPD, 59% in ZDEOPD and 65% in ZDP sequence when compared with CEOPHH. The high COD 73.5 kg/t was

found in CEOPHH bleaching effluent which reduced to 14.4 and 12.4 kg/t in bleaching effluent of ZDEOPD and ZDP sequences respectively. The significant reduction in colour and lignin content was found in DEOPD, ZDEOPD and ZDP sequences. Different studies revealed that incorporation of Z stage prior to ECF bleaching reduced the effluent load, water consumption and operating cost with same or better pulp properties compared to ECF alone (Wennerstrom et al., 2007;Carre and Wennerstrom, 2005).

Nie et al. (2014)found that AOX are generated when hypochlorous acid formed during elemental chlorine and chlorine dioxide bleaching react with lignin of the pulp. The major effect of incorporating Z stage prior to ECF bleaching was on the AOX. Reduction of 87% and 89% in AOX was achieved in sequences ZDEOPD and ZDP respectively when compared with sequence CEOPHH. The amount of AOX was also very less in comparison to DEOPD alone. The ozone bleaching cut down the generation of chlorophenolic components to significant concentration as studied byLachenal et al. (2006). Complete elimination of chlorine dioxide during bleaching may completely eliminate the AOX content of bleaching effluents (Shackford, 2003). The % reduction in different environmental parameters is depicted inFig. 8.

Z stage prior to ECF bleaching also had significant impact on the generation of chlorophenolic compounds as shown inTable 6. Chlor- ophenols are formed during bleaching by hydrolysis of chlorolignin compounds (Kringstad and Lindstorm, 1984). Ozone is an oxygen-based agent producing no chlorinated organic materials in the bleaching effluent (Fahmy et al., 2017). The total chlorophenolic compounds generated in CEOPHH bleaching sequence were 6047 mg/t which mainly belong to chlorophenols, chlorocatechols, chloroguaiacols, chlorovanillins, chlorosyringols and bromophenols.Karn et al. (2015)also found the high concentration of these compounds in paper mill effluents. Hubbe et al. (2016) found that the pulp mill effluent was characterized by high concentration of about 19 compounds of different chlorophenols (mono, di, tri, terta and penta derivatives). The amount of these compounds reduced drastically in sequence DEOPD to 472 mg/

t. The results were also reported byKaur et al. (2018a). On further incorporating the Z stage prior to DEOPD, the value of chlorophenolic compounds cut down to 58%. Mill scale use of ozone in pre bleaching stage followed by bleaching using conventional chemicals reduced generation eﬄuent volume by 28%, AOX by 78%, chlorine dioxide dose by 55% compared to use of conventional bleaching chemicals with Fig. 6.SEM images at 2500× magniﬁcation (a) CEOPHH (b) DEOPD (c) ZDEOPD (d) ZDP.

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improved pulp viscosity (Hostachy and Serfass, 2010;Sharma, 2010;

Carre and Wennerstrom, 2005).Barry (2018)also found that the use of ozone based bleaching sequences was increased in papermaking be- cause of potential reduction in eﬄuents contaminated by chlorolignin components. The % reduction in chlorophenolic compounds by family in various bleaching sequences is represented inFig. 9.

The amount of Chlorophenols reduced to 97% in ZDEOPD and 98%

in ZDP sequence in comparison to CEOPHH and 54.9% and 77% respectively when compared to DEOPD.CEPI (2013)also reported that the amount of toxic components in bleaching eﬄuents reduced to 95% due to incorporation of ECF and TCF sequences during bleaching. The pentachlorophenol was not generated in sequence ZDEOPD and ZDP.

The other Chlorophenols like 2,5,6-TCP and 2,4,5-TCP were generated in very low concentration in these two sequences. Chloroguaiacols and chlorocatechols are compounds formed in higher concentration and remain bounded to chlorolignins (Van den Berg et al., 2006). In- corporation of Z reduced the amount of chlorocatechols to more than

90% in comparison to CEOPHH. The amount of chlorocatechols was cut down to 40% in ZDEOPD and 70% in ZDP in comparison to DEOPD. The 4,5-DCC was not found in the bleaching eﬄuents of ZDEOPD and ZDP.

The other chlorocatechols such as 4-CC, TCC and 3,5-DCC were generated in meagre amount.

The chloroguaiacols, chlorovanillins and chlorosyringols were eliminated to about 99% in sequences ZDEOPD and ZDP in contrast to CEOPHH. About 60% of reduction in chloroguaiacols was found in ZDEOPD and 92% in ZDP when compared with ECF alone. 5-CG, 6-CG, 3,5-DCG and 4,5,6-TCG were not found in ZDEOPD and ZDP sequences whereas TCG was found to be in very minute quantity. Out of all the chlorovanillins studied, TCV and 4-CV present in (CEOPHH and DEOPD) were vanished in Z based sequences. The results were in close proximity of the results byKaur et al. (2017b)on oxygen delignification of rice straw bleaching. A potential impact was also found on generation of chlorosyringols, the total amount generated was 2.41 mg/t in ZDEOPD and 1.2 mg/t in ZDP. The amount of bromophenols also reduced Fig. 7.Pollution load of wastewater at each stage of bleaching sequences (C representing the composite effluent; I, II, III and IV denoting the 1st, 2nd, 3rd and 4th stage of different bleaching sequences).

Table 5

Quality of composite eﬄuent for diﬀerent bleaching sequences.

Total solids (mg/L) 5000 ± 70 3800 ± 62 2120 ± 35 1870 ± 31

Total dissolved solids (mg/L) 4400 ± 75 3120 ± 50 1790 ± 30 1540 ± 27

Total suspended solids (mg/L) 600 ± 20 390 ± 10.4 195 ± ± 2.7 178 ± 2.4

COD (kg/t) 73.50 ± 1.6 38.4 ± 0.9 14.4 ± 0.5 12.4 ± 0.5

BOD (kg/t) 26.6 ± 2.6 13.9 ± 1.4 5.2 ± 0.9 5.19 ± 0.8

TOC (kg/t) 23.8 ± 1.7 12.6 ± 0.9 6.4 ± 0.7 5.8 ± 0.6

Colour (PCU) 1435 ± 13 648 ± 8 175 ± 13 162 ± 13

Lignin (kg/t) 10.6 ± 0.1 – 4.5 ± 0.3 – 1.1 ± 0.01 – 0.9 ± 0.01 –

AOX (kg/t) 2.79 ± 0.04 – 0.96 ± 0.04 – 0.36 ± 0.01 – 0.32 ± 0.01 –

Where UR represents the uncertainty range of environmental parameters at a conﬁdence level of 95%.

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drastically in Z based sequences. The ZDP sequence was found to be the most environmentally sound as far as reduction in pollution load of bleaching effluent was concerned. Use of peroxide during bleaching inplace of elemental chlorine and chlorine dioxide was found to be more ecofriendly (Bankeeree et al., 2014). The ZDP sequence was also adopted by different mils like ITC, Bhadrachalam, India (Kumar, 2009) and Nippon papers, Yufutuc, Japan (Hostachy and Serfass, 2010). On incorporating ozone, significant improvement in the effluent characteristics was observed specially in terms of reduction in chlorolignin compounds which are the potential hazard caused by the conventional bleaching methods based on chlorine and its compounds.

3.7. Cost comparison of the bleaching sequences

The total cost evaluation of the bleaching process modiﬁcation was

carried out by comparing the cost for diﬀerent bleaching chemicals used for various sequences. Assuming the cost of bleaching chemicals, chlorine dioxide, sodium hydroxide, hydrogen peroxide and oxygen gas in kg/t in India as Rs 120, 40, 50, 9, the total cost for CEOPHH and DEOPD bleaching sequence was found to be Rs 1445 and 3952. The cost of the ozone may vary depending upon capacity of ozone generator installed in a particular pulp and paper mill. In ZDEOPD sequence, assuming the cost of ozone in kg/t as Rs 100, the cost involved for the sequence was Rs. 6346 with comparable properties to the DEOPD sequence and reduction in eﬄuent load like 63% BOD, 62% COD, 66%

AOX and 55% chlorophenols in comparison to the DEOPD. The total cost for ZDP sequence was Rs 6250 and found to be the most ecofriendly among all sequences studied as more than 90% reduction in chlor- ophenolics with the pulp brightness of 85.7% ISO was achieved in comparison to chlorine bleaching. The task of proﬁtable value becomes complicated when any process gives the elusive beneﬁts also. In these circumstances, the economic gains can be addressed on the basis of the environmental performance and conformity with the standards set by Control Boards.

4. Conclusions

Use of rice straw (agro waste) in papermaking is a sustainable ap- proach for paper industry as well as for waste management. Elemental chlorine based bleaching practices by Indian mills pose greater threats to water and pulp quality. Elemental chlorine free bleaching using chlorine dioxide can reduce the waste water load to a great extent.

Addition of ozone stage prior to ECF has significant impact on de- creasing the BOD, COD, AOX, colour and lignin content of bleaching effluent comparison to conventional process. In both the sequences, ZDEOPD and ZDP brightness of∼85% was achieved which was higher than the conventional bleaching processes CEOPHH and DEOPD. The significant reduction of 62% and 68% in COD was found in ZDEOPD and ZDP sequences in comparison to ECF alone. Reduction of 63% in BOD was achieved in ZDEOPD and ZDP sequences. The amount of AOX was also reduced to 66% and 70% ZDEOPD and ZDP sequences in Fig. 8.Reduction (%) in environmental parameters in composite effluent of

diﬀerent bleaching sequences.

Table 6

Amount of various chlorophenolic compounds present in composite eﬄuent of diﬀerent bleaching sequences.

Compounds Retention time (min) Extraction eﬃciency (%) Organohalides concentration (mg/t)

CEOPHH DEOPD ZDEOPD ZDP

3-chlorocatechols (3-CC) 5.75 104 69.2 ± 0.5 28.0 ± 0.6 18.1 ± 0.4 8.57 ± 0.3

5-chloroguaiacols (5-CG) 6.97 54 176.1 ± 0.7 2.0 ± 0.1 – –

6- chloroguaiacol (6-CG) 7.20 54 367.5 ± 0.9 – – –

2,6-dichlorophenol (2,6-DCP) 7.78 64 46.6 ± 0.3 12.4 ± 0.5 4.12 ± 0.-3 1.59 ± 0.1

2,6-dibromophenol (2,6-DBP) 8.17 68 39.3 ± 0.4 8.7 ± 0.4 2.10 ± 0.2 1.01 ± 0

2,5,6-trichlorophenol (2,5,6-TCP) 8.33 52 167.7 ± 0.6 5.9 ± 0.3 2.66 ± 0.1 1.27 ± 0.1

3,5-dichlorocatechol (3,5-DCC) 8.52 80 66.3 ± 0.4 6.7 ± 0.2 2.45 ± 0.4 1.10 ± 0.1

3,4-dichlorocatechol (3,4-DCC) 8.88 43 160.4 ± 0.5 79.0 ± 0.6 61.3 ± 0.8 29.2 ± 0.7

3,5-dichloroguaiacol (3,5-DCG) 9.73 61 43.0 ± 0.7 0.5 ± 0.1 – –

4-chlorocatechol (4-CC) 9.82 84 395.7 ± 1.2 11.8 ± 0.4 3.94 ± 0.3 0.29 ± 0

4-chlorovanillin (4-CV) 10.58 11 193.7 ± 0.7 7.9 ± 0.3 – –

2,3,4,5-tetrachlorophenol (2,3,4,5-TCP) 10.78 11 961.7 ± 1.7 74.3 ± 0.4 34.6 ± 0.7 18.8 ± 0.6

2,4,5-trichlorophenol (2,4,5-TCP) 10.84 77 131.7 ± 0.8 6.7 ± 0.3 3.73 ± 0.1 1.34 ± 0.1

5-chlorovanillin (5-CV) 11.38 40 12.6 ± 0.5 3.1 ± 0.1 1.56 ± 0.1 0.68 ± 0

4,5-dichlorocatechol (4,5-DCC) 11.56 48 6.1 ± 0.2 1.5 ± 0.01 – –

2,4,6-tribromophenol (2,4,6-TBP) 11.81 68 113.4 ± 0.9 32.4 ± 0.4 14.4 ± 0.6 8.2 ± 0.4

3,4,5-trichlorovanillin (3,4,5-TCV) 11.91 75 52.3 ± 0.7 16.3 ± 0.7 13.0 ± 0.3 6.02 ± 0.4

4,5,6-trichloroguaiacol (4,5,6-TCG) 13.08 93 214.1 ± 0.4 16.1 ± 0.2 4.11 ± 0.2 –

4,6- dichloroguaiacol (4,6-DCG) 13.17 42 720.2 ± 0.9 17.0 ± 0.3 11.4 ± 0.5 3.25 ± 0.3

3-chlorosyringol (3-CS) 13.84 102 399.2 ± 1.7 13.5 ± 0.7 – –

Pentachlorophenol (PCP) 14.50 59 103.6 ± 0.7 6.8 ± 0.3 – –

5,4-dichlorovanillin (5,4-DCV) 14.80 51 431.0 ± 0.9 64.3 ± 0.3 13.4 ± 0.4 5.73 ± 0.2

Tetrachloroguaiacol (TCG) 14.94 51 156.9 ± 0.9 5.8 ± 0.3 1.05 ± 0.1 0.45 ± 0

3,5-dichlorosyringol (3,5-DCS) 15.22 77 27.6 ± 0.6 1.2 ± 0.1 – –

Tetrachlorocatechol (TCC) 16.24 45 187.7 ± 0.8 6.7 ± 0.3 2.57 ± 0.3 1.29 ± 0.2

2,6-dichlorosyringaldehyde (2,6-DCSA) 18.73 77 396.4 ± 0.5 22.7 ± 0.7 2.41 ± 0.2 1.16 ± 0.1

Tetrachlorovanillin (TCV) 20.36 31 407.2 ± 0.6 20.7 ± 0.8 – –