Removal of sulfide and recycling of recovered product from tannery lime wastewater using photoassisted-electrochemical oxidation process

Journal Pre-proof Removal of sulfide and recycling of recovered product from tannery lime wastewater using photoassisted-electrochemical oxidation process Hosimin Selvaraj, Priyadharshini Aravind, Hema Sindhuja George, Maruthamuthu Sundaram

PII:

S1226-086X(18)31287-5

DOI:

https://doi.org/10.1016/j.jiec.2019.11.024

Reference:

JIEC 4866

To appear in:

Journal of Industrial and Engineering Chemistry

Received Date:

3 November 2018

Revised Date:

6 September 2019

Accepted Date:

17 November 2019

Please cite this article as: Selvaraj H, Aravind P, George HS, Sundaram M, Removal of sulfide and recycling of recovered product from tannery lime wastewater using photoassisted-electrochemical oxidation process, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.11.024

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Removal of sulfide and recycling of recovered product from tannery lime wastewater using photoassisted- electrochemical oxidation process

HosiminSelvaraja*, PriyadharshiniAravinda, HemaSindhujaGeorgeb, MaruthamuthuSundarama

aAcademy

of Scientific and Innovative Research (AcSIR), CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi 630 003, India. bDepartment

of Chemistry, Jamal Mohamed College, Trichy-620020, India.

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Graphical Abstract

Highlight

Sulfide and COD were removed effectively in tannery lime wastewater.

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Significant removal of COD (92%) was achieved in PEO process.

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Significant conversion of sulphide to sulphate in PEO process.

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The mixed salt (55 % Cl-and 45% SO42-) is recovered and reusing.

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

Abstract

1

Investigation and optimization of photoassisted electro-oxidation process for removal of high concentration of sulfide and COD from tannery lime wastewater was done. Electro oxidation process with UV-light were evaluated with different current densities such as 15, 20, 25 mA/cm2 with cylindrical electrodes (MMO and Ti sheet). 100% sulfide, 92% of COD and 70% total organic carbon (TOC) were effectively reduced by the current density of 25mA/cm2 in the photo-assisted electrochemical oxidation process. In Electro-oxidation process alone, the COD and TOC were not reduced effectively. Water/mixed salt (47%

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Na2SO4 and 53% NaCl) were recovered and reused for dye fixation process.

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Keywords: Tannery lime wastewater, Sulfide, Sulfate, Chemical oxygen demand, TOC

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1. Introduction

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The major components of the tannery effluents include sulfide, chromium, volatile organic compounds, large quantities of solid waste, suspended solids etc. are inducing a high

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risk of toxicity to human beings, plants and animals[1,2]. The tannery waste causes noxious effect and creates a great challenge to treat these wastes by easily adaptable and harmless

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methods. The soaking effluent contains 2- 4 % chloride and high concentration of organic content (COD: 2260-4000 mg/L) which was treated by combinedbiological and

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electrochemical processes for COD reduction[3,4]. Besides, tannery effluent was also treated by many other processes such as chemical coagulation[5],nano filtration[6], electro Fenton[7], electro oxidation of tannery wastewater[8], electro coagulation and electro dialysis[9], electro flotation[10] photo Fenton[11]etc. Lime wastewater (wastewater coming out during liming process) contains more amount of dissolved sulfide with bad odour, chloride, chemical oxygen demand etc. In most 2

of the common treatment plant, lime wastewater is mixed with the equalizer. Sengil et al reported lime wastewater treatment process using electro coagulation process in which aluminium/mild steel electrodes were used as anode[12]. 82 % of COD and 90 % sulfide were removed in acidic conditions of pH 3 where sludge formation was noticed in the process[13]. Khambhaty et al [14] isolated the halo tolerant alkalophile bacteria (pH about 9.0 to 13.6) from the lime wastewater itself which was used for reduction of COD level (65%). Besides, tannery wastewater was treated by photolysis process which did not reduce

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the physicochemical parameters like COD, sulfide content etc in the effluent. [7] An additional catalyst such as hydrogen peroxide and Fe2+ ions are needed for enhancing the

photo fenton reactions[15]. This process effectively reduced the COD (90%) and removed

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50% of TSS from tannery wastewater where sludge formation was the major problem.

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Sulfide (gas form) is one of the major components of the tannery effluent which causes the environmental problem and especially affects the human being, irritation of eyes

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(10 mg/L), nausea, headache (100 mg/L), unconsciousness (500 mg/L) and when exceeding the exposure limit (more than 600 mg/L), it affects the central nervous system and even leads

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to death. Recently, Selvaraj et al[16] recovered sulfur from sulfide by electrochemical membrane process to evaluate standard and flattened triple oxide coated electrodes. The

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sulfide oxidation process depends on pH where the sulfide was converted as sulfur at neutral pH and deposited on the anode surface which interrupted the electrochemical process and

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reduced the efficiency of sulfide oxidation. The electro oxidation of sulphide was examined using chloride as supporting electrolyte from geothermal brines with current efficiency of 90% and converted as sulfate[17]. Sulfuralso was recovered from industry effluent contaminated sulfate rich pond water by integrated biological and electrochemical process at neutral pH[18]. The soluble sulfidewas removed from synthetic and real domestic wastewater in high current densities using two chamber electrochemical membrane cell[19] and 3

suggested that sulfide can be oxidized by the formation of oxidants like OH•, O2 and Cl2 at anode surface in indirect electrochemical oxidation[20]. Besides, investigation on aqueous electrochemical sulfide oxidation was done by some investigators[20–23]. Many researchers have been working on the treatment of tannery wastewater but no literature hasbeenreported about the conversion of sulfide to sulfate particularly from tannery lime wastewater. Photo assisted advance oxidation process such as photochemical (Photo-Fenton (H2O2 /Fe2+/UV system) and heterogeneous TiO2photocatalysis (TiO2/UV system) are most

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promising techniques for organic matter degradation or decolouration. Generally, Photoelectro-Fenton process is generation of homogeneous OH radical which enhances the

degradation of organic matter with effectively[24]where TiO2 based photo electrocatalytic

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process generates the heterogeneous OH radical which depends upon the applied current

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density. Photo assisted electrochemical oxidation process has given a better efficiency compared to electro oxidation process[25]. Beside, photo electro oxidation process is

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expensive compare than electro oxidation process where energy consumption for UV lamp is higherwhere some types of lamps such as UVA (315–400 nm), UVB (285–315 nm) and UVC

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(<285 nm) were used for organic matter degradation process. While using > 300 nm wavelength of UV light , the energy consumption will be reduced which is near equal

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sunlight wavelength.[26]. Various photo assisted electrochemical process were used to treat the organic matter degradation such as Photoperoxy-coagulation[27].Photo-Fenton[28],

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photoelectro-persulfate[29] etc. The speciality of the PEO technique is the production of OCl• /OH• to degrade the COD effectively without any sludge [30,31].In textile treatment, organic matter (COD) degradation rate was higher in photo electrochemical oxidation when compared to electro oxidation due to residual OCl radical[32]. The formation of radical is lesser in PEO process which indicating that the COD concentration is effectively reduced in the process. 4

In the present study, electrochemical (EO) and photo-assisted electrochemical oxidation (PEO) processes were selected to reduce the COD level and convert the sulfide to sulfate in tannery lime wastewater. It is a new trend of green technology to the environment where the toxicity is much reduced. The available literature on sulfide conversion and tannery lime wastewater treatment are presented in Table. 1. To our best of knowledge, no investigations were reported on the treatment of tanning lime wastewater by electro /photo electrochemical methods. Herein, a new method was designed to convert sulfide to sulfate

assisted electrochemical method for recovery of salts. 2. Materials and methods

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2.1 Sample collection

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and reduce the COD level from the tannery lime wastewater through electrochemical/ photo

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Lime wastewater (pH - 11.06) was collected from EKM tannery, Erode, Tamil Nadu, India and stored at 4º C to avoid the undesirable biological and chemical reactions. The

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tannery lime wastewater was analysed and presented in Table. 2.

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2.2 Hardness removal of lime wastewater

Total hardness (TH) is an important factor which affects the electrochemical process

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by the deposition and insulation on the cathodic surface. Since, tannery lime wastewater contains 1150 mg/L of total hardness which was removed before the treatmentby

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electrochemical reactors. 1500 mg/L of NaOH was weighed and added into 1L of lime wastewater and stirred vigorously for 10 mins followed by addition of 1000 mg/L of Na2CO3 with continuous stirring. The hardness of the precipitate was allowed to settle down for 25 mins which was filtered using normal filter paper. Filtrated solution was characterised and presented in Table 2. The filtrated/ hardness removal solution was used as anolyte isboth EO and PEO processes. 5

2.3 Electro/ photo-electrochemical cell design Schematic view of electro/photo electrochemical cell is shown in Fig. 1. Single compartment electro/ photo electrochemical cell was fabricated with titanium (cathode) tube dimension about 4.7 cm diameter and a length of 21cm and mesh of metal mixed oxide (Ti/IrO2-RuO2-TiO2), tube dimension about 3.9 cm diameter and 21cm length. Anode of mesh was kept inside the titanium tube and the distance between the anode and cathode was about 5mm. 3cm diameter,20 cm length; λmax: 254nm of UV lamp was also placed inside the

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cell for the photoelectrochemical oxidation process. Total cell volume was about 250ml (with UV lamp) and 350ml (without UV lamp).

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2.4 Experimental procedure

400 mL of filtrated lime wastewater was re-circulated in two different types of

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processes i) electrochemical oxidation (in the absence of UV light), ii) photoelectrochemical oxidation (in the presence of UV light) process. Galvanostatic mode of constant current

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densities such as 15, 20 and 25 mA/cm2 were applied between the electrodes using DC power supply (APLAB L6405l) and monitored the overall cell voltage (V) as well as current

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(A) using K-PAS data logger (KM-784). The hardness removed filtrated lime wastewater was fed into an electrochemical cell with recirculating mode using a peristalticpump. The sample

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was collected every hour and analyzed the concentrations of hypochlorite, sulfide and

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chemical oxygen demand. pH of the lime wastewater, as well as UV-Visible absorbance peak were also monitored till the end of the process. All the experiments were repeated three times. After photo electro oxidation the solution volume was about 270 mL due to oxygen and chlorine gas evolution reaction occurred in the process. 2.5 Chemical analysis

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Chloride, hypochlorite, total hardness and calcium hardness of the lime wastewater were estimated by acid base titration. Sulfide, sulfate and sulfitewere estimated using Spectroquant®Pharo 300 Merck (Merck, Germany). Chemical oxygen demand was quantified by potassium dichromate digestion and then using a Spectroquant® test kit (Merck, Germany). The COD estimation was done with dilution factor only to avoid the interference of chloride and sulphide. The effluent was diluted (10times) with distilled water to reduce the chloride and sulphide concentration. After dilution 2.5ml of sample was taken into the COD

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sample tubes, 1.5 ml of digestion solution was added followed by the addition of 3.5ml of catalytic solution. The sample tube was kept at 150°C for 2h heat condition at

Spectroquant®TR 320 thermostat. (Merck, Germany) After cooling, COD of the sample was

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estimated using Spectroquant®Picco (COD/CSD) analyser (Merck, Germany).All

are presented.

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2. 6 Characterization techniques

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experiments were done three times and the estimation error was about 3%, the average values

UV-Vis spectroscopy analysis were carried out using Evolution 201 ™ UV-Vis

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Spectrophotometer (ThermoScientific). All the UV-Vis spectrum were obtained at scan mode with 1 nm band width. Samples were characterized using UV-Visible spectrophotometer

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between 200 to 400 nm. FT-IR (Bruker, Germany) analyses were carried out by mixing of samples condensed with KBr and formed as transparent KBr pellet and analysed between

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4000 and 400 cm-1. Total organic carbon (TOC) was estimated using TOC analyser (Shimadzu, Japan). The recovered mixed salt composition was observed by EDAX analysis using scanning electron microscope (TESCAN - SEM). 2.7 Performance of EO and PEO processes

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The performance EO/ Photo electro electrochemical processes were evaluated by the following equations: COD removal, current efficiency,energy consumption for COD removal coulombic efficiency and energy consumption for the processes. 𝑪𝑶𝑫𝒊−𝑪𝑶𝑫𝒇

=

𝑪𝑶𝑫𝒊 𝑪𝑶𝑫𝒊−𝑪𝑶𝑫𝒇

𝑪𝒖𝒓𝒓𝒆𝒏𝒕 𝒆𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒄𝒚 (%)

=

𝑬𝑪 (𝑲𝑾𝒉 𝒈−𝟏 𝑪𝑶𝑫 )

= (∆𝑪𝑶𝑫)𝑽𝒔

𝑪𝒐𝒖𝒍𝒖𝒎𝒃𝒊𝒄 𝒆𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒄𝒚 (𝑪𝑬)(%)

=

𝑬𝒏𝒆𝒓𝒈𝒚 𝒄𝒐𝒏𝒔𝒖𝒎𝒕𝒊𝒐𝒏 𝒌𝑾𝒉/𝒎𝟑

= 𝑼𝒔 ∫𝟎 𝑽𝑰𝒅𝒕

𝟖𝑰𝜟𝒕

× 𝟏𝟎𝟎

----------- (1)

× 𝑭𝑽 × 𝟏𝟎𝟎

----------- (2)

𝑽𝑰𝒅𝒕

----------- (4)

𝐂𝐎𝐃

------------ (6)

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𝐓𝐎𝐂

------------(5)

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𝒕

𝐂𝐎𝐃

𝐓𝐎𝐂˳

------------ (7)

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Carbon oxidation state (COS )= 4 – 1.5

𝒏𝑭𝑪 𝒓𝒆𝒎𝒐𝒗𝒂𝒍⁄ 𝑴 𝒏𝑭𝑪 𝒂𝒅𝒅𝒆𝒅⁄ 𝑴 𝟏

Average Oxidation State (AOS) = 4 – 1.5

----------- (3)

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𝑪𝑶𝑫 𝒓𝒆𝒎𝒐𝒗𝒂𝒍 𝒓𝒂𝒕𝒆 (%)

 COD removal % was calculated using Eq. (1) CODi – Initial, CODf- final COD in

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mg/L.

 COD removal current efficiency was calculated using Eq. (2) CODi – Initial, CODf-

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final COD in gL-1), Equivalent mass of oxygen is 8 (in g eq-1), I is current (A) 𝛥𝑡 is the electrolysis time in Sec, F is Faraday constant (96,487 C mol-1) and Vs is solution volume (L)[25][30]

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Energy consumption for COD removal was calculated using Eq. (3), in which V is the voltage (V), t is time (h),Vs is volume of the solution (L), and ∆COD is the experimental COD expressed in mg/L[33]. 8

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Coulombic efficiency was calculated using Eq. 4 F- faraday constant (96,485 ), nnumber of electron involve in the oxidation process (8 electrons), C –sulfide concentration and M- molar weight of the sulfide[19].

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Energy consumption of process was calculated using Eq. 5 Us – is volume of electrolyte (L), V- overall cell voltage, I- Applied current (A) and t is time (h)[25]

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Average Oxidation state (AOS) calculated using Eq. 6 After electrochemical process TOC and COD (mg/L) are calculated.[27] Carbon oxidation state (COS) Calculated using Eq. 7 COD is after electrochemical

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process. TOC0 is before electro chemical process.[34]. AOS and COS range between +4 to -4. Highest oxidation state of the carbondioxide is +4 and most reduction state

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of methane is -4. Before and after electrochemical process AOS/COSvalues are shown in Table. S1.

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3. Results and discussion

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The tannery lime wastewater was collected from EKM tannery industry, Erode which contains about 3080mg/L of sulfide, 4430 mg/Lof sulfite and 7475mg/L of COD. Before

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starting the EO and PEO processes, the hardness was removed by addition of 1500 mg/Lof NaOH and 1000 mg/Lof Na2CO3. The hardness of 1150 mg/L was precipitated and removed

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by adding the above chemicals (Table.2). The hardness removal process was reduced the contaminateswhere the limewater contains 7080 mg/Lof COD, 2610 mg/Lof sulfideand 5420

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mg/L of sulfite.

Tannery lime wastewater also contains lipids and proteins which possess long chain

hydrocarbon/hetero carbons. The biological organic matters involve in COD concentration in lime wastewater. In the present study, the removal of COD using the EO and PEO processes at different current density conditions such as 15, 20, 25 mA/ cm2 were studied and the course of duration was 18 h, 12 h, 7 h respectively.

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3.1 EO process 25 mA/cm2 of current density was applied between the electrodes during the course of electrooxidation, the samples were collected every one hour time interval and analysed by UV-Visible spectroscopy. UV-Visible absorbance peak at different current densities are shown in Fig. 2 (a) and Fig S2. The initial absorbance value was 3.76 Abs and wavelength

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was about 274 nm, the absorbance value increased at 1st and 2nd hour due to increase in colour intensity. Subsequently, the absorbance value decreasedupto 1.5 Abs from 3rdh

onwards and UV- Visible peak shifted to wavelength about 237nm. At end of the process, theabsorbance value was not reduced effectively in the EO processwhich clearly indicates

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that electrochemical oxidation process didn’t reduce COD concentration effectively. Fig. 3

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(a) shows the COD reduction rate in the EO process. During the electrochemical oxidation, 45 % COD concentration suddenly reduced within an hour and not reduced effectively at the

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end of the reaction at 7h. The COD removal was about 68%, 71 %, and 73 % for the

3.2 PEO process

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following current densities such as 15, 20 and 25 mA/cm2 respectively.

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UV-visible absorbance peaks were monitoredduring the PEO at different current densities which shown in Fig. 2(b) and Fig S3. COD was effectively reduced in PEO process

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in lime wastewater at 25mA/cm2current density. UV absorbance intensity value was noticedat 3.67 Abs and a peak was obtained at 274nm. After an hour, the absorbance intensity increased due to increase in colour (dark brown) of the lime wastewater. The absorbance intensity gradually decreased to 0.01Abs and the peak shifted from 274 nm to 229 nm at the end of the process where 99.5 % of peaks disappeared in UV spectroscopy. Fig. 3 (b) shows the COD reduction rate for PEO process. The initial concentration of COD in lime 10

wastewater was 7080mg/L, during the course of PEO, the COD concentration gradually reduced to 480 mg/L after 7h. 93.5 % COD removal was achieved with 26 % current efficiency. The COD removal was about 92 %, 92.9 % and 93.5% for the following current densities such as 15, 20 and 25 mA/cm2 respectively. In the EO and PEO processes, the COD reduction rate depends upon the applied current density. While increasing the current density, the COD reduction rate also increased. TOC reduction rate was disparate because low applied current density enhances the TOC

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reduction rate which means that the applied current density is indirectly proportional to the

TOC reduction rate. After finishing PEO process, TOC concentrations were about 575mg/L, 803mg/L, and 944 mg/L for 15, 20, and 25 mA/cm2 respectively, where in EO process TOC

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concentrations were 1681 mg/L, 2068 mg/L and 2268 mg/L at 15, 20,25 mA/cm2

3.3 Effect of pH onelectro oxidation

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respectively.

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The pH is more important on removal of COD and sulfide oxidation process. The pH of hardness removed lime wastewater was about 11.3 where pH was not adjusted to acid

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condition in the EO and PEO processes due to liberation of H2S gas which may affect the environment. While decreasing the pH in EO and PEO processes, foam formation was

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observed which may affect the current efficiency of the process. Hence, it was assumed that

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pH adjustment was not suitable for treatment of tannery lime wastewater for EO and PEO processes. The pH variation is shown in Fig. 4for both the PEO and EO processes with different current densitiesAravindet. al [30] also reported that acid pH was more suitable for photoelectrochemicaloxidation of textile effluent. In EO process, pH was gradually decreased from 11.30 to 8.2 for 4 h at 25mA/cm2 current density. In PEO process, pH decreased to 8 within 3h. Similarly,The variation in 11

decreasing of pH was observed with different current densities of 15 and 20 mA/cm2 in both the EO and PEO processes. At high current density, pH suddenly decreased first and increased slightly and finally continue about 8.2. It can be claimed that the COD can be reduced at alkali pH without any formation of foam. 3.4Effect of hypochlorite on COD reduction and sulfide conversion Chloride ions are used in the generation of hypochlorite and involved in indirect

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electro-oxidation to reduce the COD level in the effluent. Since the lime wastewater contains chloride about 17000 mg/L, there is no need to add supporting electrolyte for electro/

photoelectrochemical oxidation process. The concentration variation of hypochlorite is

presented in Fig. 5.The formation of free hypochlorite generation was observed during the

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EO and PEO processes. While applying current density of 25mA/cm2, the free hypochlorite

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generation rate was faster in EO process which was noticed at 2ndh and the concentration of free hypochlorite gradually increased upto 1488 mg/L at 5th h. In PEO process, at current

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density of 25mA/cm2, the free hypochlorite generation was about 1116 mg/L. It is due to consumption of hypochlorite for effective removal of COD in PEO process. When current

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density increased, the free hypochlorite rate also increased in both the EO and PEO processes. It may be assumed that the formation of radicals are consumed by the presence of

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organics [30,31]. The mechanism path way explains as follow as on COD reduction process. 𝐻𝑂𝐶𝑙 + ℎ𝑣 → 𝐻𝑂• + 𝐶𝑙 •

--------------------------------- (8)

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𝐻𝑂• + 𝐻𝑂𝐶𝑙 → 𝐻2 𝑂 + 𝐶𝑙𝑂• --------------------------------- (9) 𝑅(𝑝𝑟𝑜𝑡𝑒𝑖𝑛𝑠 𝑎𝑛𝑑 𝑙𝑖𝑝𝑖𝑑𝑠 ) + 2𝑂𝐶𝑙 • → 𝐶𝑂2 + 𝐶𝑙2 -------- (10) Fig.6 (a) shows sulfide reduction rate for both the EO and PEO processes. In the present study, different current densities were applied between the electrodes and the initial anode potential was about 700 mV vs SCE. Sulfide to sulfate conversion of oxidation 12

potential was about 200mV vs SCE[16]. Initially,sulfidegets oxidized into sulfur oxides which may be sulfate/sulfite. Hypochlorite was not observed in the first hour which was assumed that oxidation of sulfide and COD reduction may consume fully at the initial period. During the process colour offiltrated lime wastewater was changed into a dark brown colour which is due to sulfide oxidation process at alkaline pH in the presence of pollutants. It is well known that sulfide is converted into sulfur oxides like sulfate or sulfite at alkaline pH [17]. Selvaraj et al reported that neutral pH is more suitable for sulfide to sulfur conversion

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process[18]. The concentration of sulfide ions and bad odour were gradually decreased within an hour in the present study. The colour of lime wastewater colour initially increased and gradually decreased after 1h which completely disappeared at 3rd h and formed as a

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colourless solution. The colour changes and sulfide removal rate increased within the

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increasing current densities.

The presence of sulfite about 4430 mg/L/ sulfide 2610 mg/L in the filtrated lime

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wastewater was also converted intosulfate at the end of the process where the treated solution contained 9890 and 9700 mg/Lsulfate ions in electro and photoassistedelectrochemical

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oxidation respectively. The sulfate concentration was increased in this process due to volume of the electrolyte. The initial electrolyte volume was about 400 mL,oxygen and chloride

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evolution were occurred during the processwhich reduced the volume of the electrolyte about 285 mL. Hence, the sulfate concentration was high at the end of the process. The 100

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% sulfide ions were converted intosulfate ions in both the EO and PEO processes with in 3h. Pikaar et alreported that the presence of chloride did not increase sulfide removal rate during the experiments in domestic wastewater using two compartment electrochemical membrane cell[19]. They suggested that the indirect oxidation of sulfide by in situ generation of oxygen by the Ir/Ta MMO coated electrodes occurred in the conversion mechanism [20]. Waterston et al reported that indirect electro oxidation by the chloride ions involve on the oxidation of 13

sulfide[17].The present investigators support the above two mechanisms. The oxygen evaluation potential of triple oxide coated electrodes was about 400mV vs SCE. In the present study, anode potential of triple oxide coated electrode slowly increased from 700 mV to 1200 mV vs SCE during electrochemical process. Hence, it is claimed that the evaluation of oxygen, hypochlorite and radical reduced the concentration of sulfide/sulfite by indirect electro oxidation process. No precipitation/deposition of elemental sulfur occurred during the EO and PEO processes which supports with the observation made by Waterston et. al [17].

S2- + 4H2O →

SO42- + 8H+ + 8e-

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The following equations can be proposed on the mechanism of sulfide oxidation ------------- (11)

S2- (aq) + 8OCl- (aq) → SO42- (aq) + 4Cl2 (g) + 2O2 (8)

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------------- (12)

2S2- + (4H2O → 8H+ + 2O2) → 2SO42- + 8H+ + 8e-

------------- (14)

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3.5 Degradation process identification

------------- (13)

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2S2- + 6OCl- (aq) + 4OH- → 2SO42- (aq) + 6Cl- (aq) + 2H2O(l)

Table S2 shows the IR peaks value of before and after treatment of lime wastewater.

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Fig S4.shows the IR spectra of initial tannery lime wastewater, after EO and PEO processes. 3765 cm-1 and 3700 cm-1 peaks value corresponds to O-H alcohol group and 3408 cm-1

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indicates the presence of N-H group. 2916 cm-1 and 2372 cm-1 peaks were attributed to

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alkane C-H cm-1 stretching frequency. 1719 cm-1 and 1601 cm-1 represented C=O stretching as well as C=C stretching, 1348 cm-1represented that C-N stretching of secondary amine. 1090 cm-1 is C-O alcohol and 979 cm-1 for C-Cl stretching absorption peaks. Maximum of similar peaks were obtained for lime waste water for before and after treatment. In the case of PEO process, entirely different functional group peaks were noticed in treated lime

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wastewater compared to the EO. These results predict the disappearence of N-H, C-O, C-Cl stretching and CH3 which reveals that PEO has performed well in degradation process. 3.6 Effect of current density in COD reduction Fig. 6 (b) shows the current efficiency of degradation of limewater COD in both the EO and PEO processes. The current efficiency of the EO was in the range between 13.5 and 18.7 % and PEO current efficiency range between 19% and 26%. In tannery lime

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wastewater treatment, while increasing the current density, the COD degradation rate was increased in both the EO and PEO processes. The significant reduction of COD and sulfide were noticed at 25mA/cm2 of the PEO process. On the basis of COD reduction, the higher current density (25mA/cm2) was selected for further study. Coulombic efficiency for

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removal of COD in EO and PEO processes was71.5% and 93.4% respectively. Columbic

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efficiency was about 71.5% and 99.99% for removal of COD and sulfide in EO process and in PEO process, the efficiency was 93.4% for COD and 99.9% for sulfide (Table. S1). In the

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3.7 Recovery of water

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present, study while increasing the current density the degradation rate also increase.

Tannery lime wastewater was effectively treated using photo-electro oxidation

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process at 25mA/cm2 current density. The solution was condensed using BUCHI Rotavapour R3 and mixed salt was separated. Fig. 7 (a) shows the image of mixed salt and recovered

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water. The recovered water was analysed with different parameters and shown in Table. S3. The recovered water parameters of minerals concentration contained equal/low level of WHO recommended parameter concentration. Fig. 7(b) shows the spectrum of elemental concentration viz Oxygen 44. 00 %, sodium 34. 06 %, chloride 16. 86 % and sulfur 08. 08 %. The mixed salt contains 47 % of sodium sulfate and 53% of sodium chloride and other cation impurities were not observed in EDAX analysis. 15

The sulfide removal was about 100% and COD reduction was about 92% in the lime wastewater collected from a tannery plant at Erode, India. Most of the tanneries use mixed soak liquor, lime wastewater, dyeing effluent in equalizer which is coagulated by chemical addition viz. alum for the removal of organic and suspended solids. Subsequently, anaerobic treatment is done to remove COD where sulfide remain in the system which creates bad smell and affects the environment. This study recommends EO/PEO processes to reduce the sulfide/sulfite and COD concentrations in limewater. It is suggested that releasing the

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hypochlorite should be minimized to avoid the chlorine production in the system. 4. Conclusion

The present study demonstrates about the possibility of removal of sulfide and

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reduction of chemical oxygen demand (COD) from the tannery lime wastewater using both

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electrochemical and photoelectrochemical oxidation processes. 100 % of sulfide and bad odour were effectively removed from the lime wastewater using both the EO and PEO

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processes within 3h. 92% of COD removal and 26.2% current efficiency were achieved by the photoelectrochemical oxidation process but in electrooxidation 62% of COD removal

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and18 % current efficiency were obtained. The coulombic efficiency of PEO was better than EO. While increasing current densities, the COD removal rate also increased but the TOC

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removal rate was indirectly to COD reduction. The energy consumption was about 0.30 kWhL-1, 18.6kWhg-1CODand 0.33 kWhL-1, 24.33 kWhg-1COD for PEO and EO

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respectively.After treatment, uncontaminated water was recovered as well as mixed salts of 47 % NaSO4 and 53% NaCl were also recovered from theprocess.This study points out a new trend of green technology for the large-scale process in removal of sulfide and COD effectively from tannery lime wastewater. 0.031$/L was needed to treat the lime wastewater. The treatment cost can be reduced using solar electrical energy.The recovered products can be used for further process. The recovered products can be used for further process. The 16

recovered products cost value will compromise the electrical energy cost of the treatment process. Acknowledgement CSIR-HRDG, New Delhi is gratefully acknowledged for the senior research fellowship (SRF) of HosiminSelvaraj and second author (A. Priyadharshini) also thanks DST for awarding Inspire fellowship. The authors thank the Council of Scientific and Industrial

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Research (CSIR), India for sponsoring this project under Sustainable Environmental Technology for Chemical Allied Industrial (SETCA) CSC-0113. Authors are grateful to

CSRI-CECRI and Academy of scientific and innovative research (AcSIR) to carry out this

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study under Ph. D program.

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[20] P.K. Dutta,, K. Rabaey,, Z. Yuan,, J. Keller, Water Res. 42(20) (2008) 4965–75. [21] B.G. Ateya,, F.M. AlKharafi,, A.S. Al-Azab, Electrochem. Solid-State Lett. 6(9) (2003) C137--C140. [22] B.G. Ateya,, F.M. AlKharafi,, A.S. Alazab,, A.M. Abdullah, J. Appl. Electrochem. 37(3) (2007) 395–404. [23] P.K. Dutta,, R.A. Rozendal,, Z. Yuan,, K. Rabaey,, J. Keller, Electrochem. Commun. 11(7) (2009) 1437–40. [24] Y.-B. Xie,, X.Z. Li, Mater. Chem. Phys. 95(1) (2006) 39–50. [25] C.A. Mart\’\inez-Huitle,, E. Brillas, Appl. Catal. B Environ. 87(3–4) (2009) 105–45.

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[36] M. Panizza,, G. Cerisola, Environ. Sci. Technol. 38(20) (2004) 5470–5. [37] L. Szpyrkowicz,, G.H. Kelsall,, S.N. Kaul,, M. De Faveri, Chem. Eng. Sci. 56(4) (2001) 1579–86. [38] N.N. Rao,, K.M. Somasekhar,, S.N. Kaul,, L. Szpyrkowicz, J. Chem. Technol. Biotechnol. 76(11) (2001) 1124–31. [39] L. Szpyrkowicz,, S.N. Kaul,, R.N. Neti,, S. Satyanarayan, Water Res. 39(8) (2005) 1601–13. [40] L. Szpyrkowicz,, J. Naumczyk,, F. Zilio-Grandi, Water Res. 29(2) (1995) 517–24. 19

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Figure caption

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Figure. 1 Schematic and original view ofElectrochemical/ Photo-assisted electrochemical reactor.

Figure. 2Absorbance variation inUV- visible spectroscopya) EO and b) PEO process.

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Figure. 3 COD reduction rate for EO and PEO.

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Figure. 4. pH variation of the electrolysis process of EO and PEO

Figure. 5. HOCl generation in electrolysis process of EO and PEO

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Figure. 6a) Sulfide reduction rateb) Current efficiency for both EO and PEO

Figure. 7Recovered products from tannery lime wastewater a) Mixed salts b) portable water c) EDAX analysis for recovered mixed salt (Oxygen 44. 00 %, Sodium 34. 06 %, Chloride 16. 86 % and Sulfur 08. 08 %).

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Table’s caption Table. 1 Available literature on lime wastewater treatment S.No

Experiments type

Effluent

Con. S2-

Efficiency

Ref

82 % COD and 90

[12]

(mg/L) 1

Electrocoagulation

Lime

3000

wastewater 2

Biological

% Sulfide

Lime

-

65% COD reduction

[14]

3432

78 % sulfide

[16]

wastewater

5

6

7

Sulfide

(membrane cell)

oxidation

Single compartment

removal

Sodium sulfide

electro oxidation

with NaCl

Electro oxidation

Sulfide

(membrane cell)

oxidation

Electro oxidation

Domestic

(membrane cell)

wastewater

Electro oxidation

Sulfide

(membrane cell)

contaminated

343

3432

70% sulfide removal

[18]

833

85% Sulfide

[19]

removal 100

9

wastewater

Single compartment electro oxidation

10

Single compartment

11

Single compartment electro oxidation

Single compartment

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12

electro oxidation 13

Single compartment

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electro oxidation

14

Lime wastewater mixed with equalizer with

99% sulfide and

[20]

[35]

81% TDS

350

[36]

32.0

[37]

1.8-2.0

[38]

352

[39]

205

[40]

wastewater Tannery

wastewater

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electro oxidation

Tannery

7285

lP

filtration

Lime

95% sulfide removal

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Oxidation-coagulation-

[17]

efficiency

water 8

90 % current

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4

Electro oxidation

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3

Tannery

wastewater Tannery

wastewater Tannery wastewater Tannery lime

Personal observation

wastewater

Existing technology

chemical addition (anaerobic treatment )

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Single compartment electro oxidation

Lime

3040

wastewater

93% COD and 100 % sulfide removal

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Present study

Table. 2 Tannery lime wastewater characterization S.No

parameters

Collected lime wastewater (mg/L) *

After After After Photohardness electrochemical electrochemical removal of oxidation oxidation lime (mg/L) * (mg/L) * wastewater (mg/L) * 11.03 8.30 8.43

pH

11.03

2

0.02

0.04

2.70

5.50

3

Dissolved Oxygen COD

7475

7080

2000

480

4

TOC

2942.5

2815

2531

944

5

32.8

45.8

53.2

32.0

6

Conductivity (mS/cm) Alkalinity

4800

6800

-

-

7

Phosphate

38

24.5

1.2

1.32

8

Sulphate

149

149

9890

9700

9

Sulfide

3080

2610

-

-

10

Sulphite

4430

5420

-

-

11

Chloride

17000

13400

12762

10635

12

Iron

0.16

0.29

0.06

0.12

13

Total hardness

1150

50

50

50

14

Calcium hardness Magnesium hardness Total dissolved solide

1000

50

50

50

150

-

-

-

41200

51500

54100

49200

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