Os resultados obtidos mostraram que a troca de eletrodos só tem efeito significativo para o tratamento associado ao eletrólito e a densidade de corrente
A densidade de corrente foi determinante pra eficiência de remoção de compostos orgânicos nas correntes mais altas (50%), mas também foi possível verificar que em correntes baixas há uma remoção significativa desses compostos na presença de K2SO4 (30-35%).
Fazendo uma comparação entre os tratamentos propostos foi possível verificar que eletrodo de Ti/Pt, na presença do eletrólito K2SO4 se mostrou mais eficiente para a remoção de compostos orgânicos. Entretanto, o eletrodo de IrO2 é mais eficiente na presencia de cloreto, devido a produção de cloro ativo.
Objetivo principal do estudo foi atingido uma vez que foi possível comprova a existência de compostos orgânicos no efluente, bem como foi possível comprovar a remoção desses compostos em até 99,36%. Conseguindo assim deixar o efluente dentro dos limites permitidos pela regulamentação de descarte de efluentes.Por fim concluímos que os POAS estudados nesta tese foram eficientes no tratamento contaminantes fenólicos e orgânicos. Dentre as metodologias de tratamento do efluente da Indústria de Beneficiamento Castanha de Caju, o material eletródico que apresentou o melhor desempenho na remoção da DQO e na degradação dos compostos fenólicos foi o ânodo não-ativo de DDB, em todas as densidades de corrente aplicadas. Além disso, o DDB têm a vantagem de não passivação do eletrodo.
Para ambos os tratamentos propostos nos dois artigos dessa tese, os compostos fenólicos foram reduzidos à concentrações abaixo do limite estabelecido pela Resolução 430/2011 do CONAMA em até 2 horas de oxidação, mostrando que o tratamento eletroquímico é uma técnica efetiva na remoção destes contaminantes nas condições operacionais utilizadas, sendo eficiente quando analisado do ponto de vista ambiental.
Entretanto, as concentrações de nitrato e fósforo total, mesmo tendo sido significativamente reduzidos, ficaram acima do estabelecido pela legislação brasileira
113 para o descarte do efluente em corpos receptores. Porém, os resultados indicam que o efluente tratado pode vir a ser utilizado em irrigação de campos de girassóis para a produção de biodiesel, uma vez que nitrogênio e fósforo são macronutrientes importantes para a nutrição das plantas.No artigo do Capitulo 2 dessa tese, o estudo do
lodo dos experimentos de eletrocoagulação a 8,33 e 100 mA cm-2 revelou uma
morfologia heterogênea com placas laminares interconectadas, englobando oxigênio, carbono e ferro como elementos principais. O sugere o aproveitamento desse resíduo na indústria da construção civil em tijolos e cerâmicas em geral.
Considerando somente o consumo energético necessário para a remoção dos compostos orgânicos do efluente de forma a atender a Resolução 430/2011 do CONAMA, o menor valor foi obtido utilizando o eletrodo de Fe a uma densidade de
corrente aplicada que após 15 min de tratamento, alta para o consumo de 15,0 kWh m-3
de energia.
Os resultados mostram a eficiência de se utilizar eletrodo de DDB notratamento eletroquímico do efluente da Indústria de Beneficiamento Castanha de Caju. Porém, estudos mais aprofundados devem serrealizados, incluindo análises dos subprodutos gerados, de forma adeterminar a real eficiência da metodologia desenvolvida.Assim, o sistema EC8.33 + EO foi considerado o mais eficiente e viável para tratar o efluente real da castanha de caju. A evolução dos ácidos carboxílicos presentes no sistema foi seguida, demonstrando a possibilidade dos compostos serem facilmente gerados / eliminados ao longo do tempo de tratamento.
A utilização de energia solar tem um elevado efeito sinergético e econômico, e esta fonte renovável tem que ser mais aproveitada, principalmente em países subdesenvolvidos que sofrem com a seca e tem limitações nos recursos hídricos. Então as perspectivas para um futuro próximo é desenvolver plantas pilotos solares autônomas para descontaminação de águas em larga escala de efluentes reais. Além disto, pesquisar novas tecnologias que atendam todos os requisitos para aplicação em dispositivos descentralizados. Por fim, é importante considerar que as pesquisas que priorizam a valorização e a preservação do meio ambiente, em especial dos recursos hídricos, devem ser recolocadas na academia como condição necessária não apenas para o
114 desenvolvimento sustentável das sociedades, mas como ferramenta essencial para a própria existência da vida.
Journal of The Electrochemical Society, 165 (13) E659-E664 (2018) E659 0013-4651/2018/165(13)/E659/6/$37.00©The Electrochemical Society
Cashew-Nut Effluent: An Anodic Oxidation Treatment Using a
Batch Recirculation Reactor with BDD Anode
Patr´ıcia R. F. da Costa,1Emily C. Tossi de A. Costa,1Suely S. L. Castro, 2
Carlos A. Mart´ınez-Huitle, 1,3,zand Ana S. Fajardo 1,4,z
1Institute of Chemistry, Federal University of Rio Grande do Norte, Natal, Rio Grande do Norte P59078-970, Brazil 2Faculty of Exact and Natural Sciences, State University of Rio Grande do Norte, Campus Central, Mossor´o
P59625-620, Rio Grande do Norte, Brazil
3Institut f¨ur Organische Chemie, Johannes Gutenberg-Universit¨at Mainz, P55128 Mainz, Germany
4Postgraduate Program in Petroleum Engineering Science, Center for Exact and Earth Sciences, Federal University of
Rio Grande do Norte, Natal, P59078-970 Rio Grande do Norte, Brazil
A BDD-batch reactor with recirculation is presented, for the first time, to the electrochemical treatment of an actual cashew-nut effluent, which is a real environmental problem in the Northeast region of Brazil. This system was characterized in terms of mass
transfer coefficient (km= 2.22 × 10−5m s−1), which led to a limiting current intensity of 1.91 A. Different parameters, such as,
current density, kind of electrolyte and initial pH were evaluated considering the chemical oxygen demand removal, current efficiency, energy consumption, by-products generation and toxicity. The results clearly indicated that the addition of external electrolyte and the change of initial pH are two factors that do not lead to a significant improvement in the degradation of organic matter when compared to the treatment of the effluent as received. Conversely, an increase in current density promoted a COD removal up to 89%, after 150 min of oxidation, due to the enhancement of the generation rate of strong oxidants. Low production of carboxylic acids was also detected over time. Additionally, it seems that the treated effluent can be used to irrigate specific crops, like those applied to the production of renewable fuels, based on the toxicity tests performed.
© 2018 The Electrochemical Society. [DOI:10.1149/2.0121813jes]
Manuscript submitted May 16, 2018; revised manuscript received August 15, 2018. Published September 18, 2018.
In Brazil, cashew-nut (Anacardium occidentale) production is quite centered in the northeast region, being processed almost 99% of 108 thousand tons of product per year.1 This region stands out,
due to the adaptive capacity of the cashew tree to low fertility soils, high temperatures and water stress, common in semi-arid areas.2In
the off-season of the annual crops, cashew becomes important for the generation of jobs both in the field and in industries.3 How-
ever, cashew-nut processing results in the generation of hazardous waste for the environment. The shelling operation produces outer shell waste which is about 70% of the original raw nut weight and contains 20–25% of oil named cashew-nut shell liquid (CNSL). This liquid may be classified as natural or technical, depending on the ex- traction method. The natural CNSL is extracted with solvents, being its main components: anacardic acid (60–65%), cardol (10–20%) and cardanol (10%). In its turn, technical CNSL is obtained burning the nuts industrially at high temperatures, containing cardanol (60–65%), cardol (15–20%), polymeric material (10%) and small amounts of metilcardol.4,5This oil is rich in organic matter encompassing phenolic
compounds with antioxidant properties, being also toxic and possess- ing mutagenic potential.6–9Thus, effluents from cashew-nut industries
are not suitable to be treated by biological processes, since they con- tain recalcitrant compounds, which inhibit microorganisms’ action.10
Due to this restriction, electrochemical advanced oxidation processes (EAOPs) emerge as promise alternatives to eliminate a broad-range of organic contaminants from wastewaters. These techniques promote the electrogeneration of strong non-selective oxidizing species as the hydroxyl radical (•OH).11,12Among EAOPs, the most used is the elec-
trochemical oxidation (EO), in which two oxidation mechanisms can be followed, direct and/or indirect.13In the former one, pollutants are
destroyed directly at anode’s surface (M) by electron transfer, yielding very poor decontamination. Whereas, in the indirect EO, contaminants are degraded at bulk medium by electrogenerated oxidants, such as
●OH radicals (Eq.1), active chlorine species (Cl
2(aq)– Eq.2, HClO –
Eq.3and ClO─– Eq.4) and peroxydisulfate (Eq.5).11,12
M+ H2O→ M
•
OH+ H++ e− [1]
2Cl−→ Cl2 (aq)+ 2e− [2]
Cl2 (aq)+ H2O→ HClO + Cl−+ H+ [3] zE-mail:[email protected];[email protected]
HClO →← H++ ClO− [4]
2SO24−→ S2O28−+ 2e− [5]
Regarding anode materials, the boron-doped diamond (BDD) is an example of a non-active anode used in EO that does not provide any catalytically active site for the adsorption of reactants and/or products in aqueous media.11This material provides satisfactory treatment ef-
ficiencies for difficult wastewaters, since BDD(●OH) radicals formed from water electrolysis on its surface are responsible for the electro- chemical mineralization of pollutants (Eq.6).11,12
BDD•OH+ R → BDD + mCO2+ nH2O+ xH++ ye− [6]
Taking into account that, the scale up from the lab to the full scale is an issue of major importance in electrochemical technology as well as the efficient removal of recalcitrant compounds from water using BDD anodes in pilot plants,12,14–17the development of electrochemical
water reactors to depollute industrial effluents is an important insight that must be explored. In this context, the novelty of this work is based on the depuration of a real effluent from the abovementioned industry using, for the first time, a BDD-batch reactor with recirculation and it should be highlighted that, to the best of our knowledge, no attempts have been published about the scale up of this process yet. Only the electrochemical treatment of the cashew-nut effluent as well as the individual components of this kind of wastewater (as single organic compounds), have already been investigated,18 in batch conditions
using electrocatalytic materials in reduced geometrical dimensions.
Experimental
Real effluent.—An actual wastewater was collected from a cashew-nut processing industry located in the northeast Brazilian re- gion (Rio Grande do Norte) and the experiments were performed with previously filtered effluent. The filtration stage was necessary in order to avoid electrode fouling during the treatment process.19,20
Wastewater physicochemical characteristics are displayed in TableI. As it can be observed, the effluent before and after filtration has a neutral character (7.20–7.22), high chemical oxygen demand (COD = 962–1134 mg O2L−1), high chloride concentration (668–709 mg
L−1) and its conductivity was around 4.32–4.77 mS cm−1. Addition- ally, the amount of solids was determined, with 1864 mg L−1 and
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E660 Journal of The Electrochemical Society, 165 (13) E659-E664 (2018)
Table I. Physicochemical parameters of an actual effluent of a cashew-nut processing industry.
Actual effluent Actual effluent
Parameter (Before filtration) (After filtration)
pH 7.2± 0.060 7.2± 0.060 Conductivity (mS cm−1) 4.8± 0.10 4.3± 0.10 COD (mgO2L−1) 1134± 10 962± 95 TS (mg L−1) 5118± 256 - TSS (mg L−1) 1864± 76 - TDS (mg L−1) 3410± 214 - Chloride ions (mg L−1) 709± 29 668± 6 Sulfate ions (mg L−1) 64± 0.5 16± 0.5 Nitrate ions (mg L−1) 8± 2 2± 2
3410 mg L−1 of total suspended solids (TSS) and total dissolved solids (TDS), respectively.
Electrochemical system and procedure.—Electrochemical elec- trolyses were accomplished in a batch recirculation system,21during
150 min at 25◦C. In each experiment, 1 L of the real effluent was recirculated through the electrolytic cell by a pump at 4.61 m3s−1. A
Nb/BDD anode (METAKEM GmbH – Germany) and a Ti cathode, both disks with 63.6 cm2of geometric area, were placed at 1.2 cm of
distance. These electrodes were linked to a Politerm POL-16 power supply and the applied current density ranged from 15.5 mA cm−2 to 35.5 mA cm−2. Some experiments were performed without elec- trolyte, while in others, the effect of this parameter was studied adding salts to the effluent, such as 1.5 g L−1 NaCl (Vetec) or 1.4 g L−1 Na2SO4(Anidrol). The initial pH values were varied between 3 and
10, adjusted with H2SO4(CRQ Qu´ımica) and NaOH (CRQ Qu´ımica),
whenever necessary. Conductivity and pH parameters were followed by a HI 4321 HANNA conductivity meter and HI 4224 HANNA pH meter, respectively; but not adjusted during the treatment time. Addi- tionally, before use, Nb/BDD material was polarized during 30 min with 1 L of 0.1 M H2SO4solution at 30 mA cm−2to remove any kind
of impurity on its surface.
Analytical techniques.—In order to evaluate the evolution of the treatment process, samples were withdrawn over time and then ana- lyzed by analytical techniques at least in duplicate to minimize exper- imental errors. The maximum deviations were 5% for each technique. The determination of total dissolved solids (TDS) and total sus- pended solids (TSS) was carried out according to the Standard Meth- ods, techniques 2540C and 2540D, respectively.22 The degradation
of the effluent was monitored by chemical oxygen demand (COD) according to the closed reflux system of the Standard Methods (5250D).22 Samples were digested in a HI839800 HANNA ther-
mal reactor at 150◦C during 120 min, and then, the absorbance was measured at 610 nm in a HI83399 HANNA photometer. Stan- dard solutions of potassium hydrogenphthalate (Sigma-Aldrich) were prepared to construct the COD calibration curve within the range 0–1200 mgO2 L−1. After obtaining COD values, current efficiency
(CE) and energy consumption (EC) parameters were calculated in terms of % and kWh m−3according to Eq.7and Eq.8, respectively.
CE (%)= (COD)expFVs
8It × 100 [7]
EC kWh m−3= EcellIt 1000Vs
[8] where(COD)exp corresponds to the COD removed (g O2 L−1), F
to the Faraday constant (96,487 C mol−1), Vs to the solution vol-
ume (L for current efficiency and m3 for energy consumption), 8 to
the equivalent mass of oxygen (g eq−1), I to current intensity (A), t to the electrolysis time (s for current efficiency and h for energy consumption) and Ecellto the average potential difference of the cell
(V).
The characterization of the recirculation cell was performed by the technique described by Quiroz et al.23 and Canizares et al.24in
order to interpret the effects observed during the electrochemical treatment by the hydrodynamic conditions as well as the limiting current. The mass transfer characterization of the electrochemical cell was obtained by polarization curves using different aqueous solutions of potassium ferro/ferri-cyanide (K3Fe(CN)6/K4Fe(CN)6)
within the range of 20 to 60 mM in 0.5 M NaOH. From those curves were obtained the limiting current values at different redox couple concentrations, allowing the determination of the mass-transfer coefficient (km– m s−1) by Eq.9.23,25
km =
ilim
z F A Corg
[9] where ilimis the limiting current intensity (A), z is the number of elec-
trons involved in organics mineralization reaction, A is the area of the anode (m2), and C
orgis the concentration of organics in the solution in
mol m−3.
Chloride ions concentration was determined by the Mohr method, as described by Brito et al.26The concentration of carboxylic acids
(acetic, formic and oxalic) was evaluated by HPLC analysis, where 10μL of sample was injected into a DIONEX system (LC Ultimate 3000) via an autosampler (Ultimate 3000). The mobile phase consist- ing of 100 mM of Na2SO4at pH 2.65 (adjusted with methanesulfonic
acid) was eluted at 600μL min−1 during 15 min in an Acclaim Or- ganic Acids column (Acclaim OA, 5μm, 120 Å, 4.0 × 250 mm) at 25◦C. The detection was accomplished at 210 nm in a diode ar- ray detector (Ultimate 3000 DAD). Nitrate and sulfate concentrations were determined by ionic chromatography analysis, where 10μL of sample was injected into Dionex ICS 2000 with Dionex DS6 con- ductimetry detector via an autosampler (AS40 Automated Sampler). The mobile phase was composed by 10–50 mM of KOH and eluted at 250μL min−1 during 32 min in a ThermoFisher Scientific AS19 column (250 mm× 2 mm) at 30◦C. The retention time was 16.9 and 21.3 min for NO3—and SO42—, respectively. Acute toxicity tests were
performed with lettuce seeds (Lactuca sativa), during 120 h without light, according to Sobrero et al.27 The phytotoxic effects of pure
compounds or complex mixtures can be evaluated by the germination process of the seeds during the first days of growth. The germination index (GI) was expressed according to Eq.10.28–30
GI (%)=
number of seeds germinated in sample number of seeds germinated in control
×
average sum of root lengths in sample average sum of root lengths in control
× 100 [10]
Results and Discussion
Mass transfer coefficient.—At Fig.1aare presented the polariza- tion curves obtained for the mass transfer characterization of the elec- trochemical cell with different concentrations of potassium ferro/ferri- cyanide solutions (20 to 60 mM in 0.5 M NaOH). The values of 225 mA, 575 mA and 810 mA of limiting current intensity were achieved for 20 mM, 40 mM and 60 mM K3Fe(CN)6/K4Fe(CN)6so-
lutions, respectively. These limiting values were plotted against the ferro/ferri-cyanide concentrations (Fig.1b). The slope of the limiting current intensity (ilim)/[K3Fe(CN)6/K4Fe(CN)6] led to the calculation
of the experimental mass transfer coefficient (km) by Eq.9, which is
km= 2.22×10−5m s−1. That value was endorsed to estimate the lim-
iting current (ilim) in order to establish electrical operating conditions
for the treatment of an industrial effluent. Generally, a real wastewater contains a wide variety of pollutants without a well-known concen- tration, then, the ilimcan be estimated from the initial value of COD
using the Eq.11.25
ilim = 4 A F kmCOD [11]
where, 4 is the number of exchanged electrons and COD is the chemical oxygen demand in mol O2 m−3. The limiting current
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Figure 1. (a) Polarization curves for the mass transfer characterization of the electrochemical cell, using the ferro/ferri-cyanide redox couple at ( ) 20 mM, ( ) 40 mM and ( ) 60 mM in 0.5 M NaOH. (b) Dependence of the limiting current on the ferro/ferri-cyanide concentration.
estimated was 1.91 A. That current allows understanding the oxidation conditions (charge or mass transport control) in which the process was performed.
Effect of current density on treatment efficiency.—Fig. 2
shows the effect of the applied current density (15.5, 25.5 and 35.5 mA cm−2) on the elimination of organic matter of the cashew-nut effluent (COD removal, in %), and current efficiency values achieved during the galvanostatic electrolyses of the wastewater. As it can be observed in Fig.2a, an increase on the j favors a rapid COD removal over time. After 150 min of electrolysis, 49%, 74% and 89% of COD was removed by applying 15.5, 25.5 and 35.5 mA cm−2, respec- tively. This performance is related to the enhancement of the genera- tion of•OH radicals (Eq.1) or production of other oxidizing species (Eq.2–Eq.5). However, as shown in Fig.2b, it is also interesting to observe the particular trend of CE (Eq.7) obtained for all the applied current densities. For 25.5 and 35.5 mA cm−2, the current efficiency was almost constant at 80% and 100%, respectively, during the first 60 min of treatment. After that, CE values gradually decreased un- til 60% of efficiency. This behavior can be considered characteristic of electrolyses under current control, for 25.5 and 35.5 mA cm−2.31
Theoretically speaking, these experiments are performed applying a current lower than the limiting one (iappl < ilim). However, in this
work, not all the currents applied (0.99, 1.61 and 2.26 A) were lower than ilim(1.91 A). Subsequently, the process was under mass-transport
control where secondary reactions (Eqs.12–14) were involved, result- ing in a CE decrease. For 15.5 mA cm−2, the current efficiency was
Figure 2. (a) COD removal and the (b) current efficiency over time of the electrochemical oxidation of 1 L of the cashew-nut effluent with Nb/BDD
anode at pH 7.0, under different current densities: ( ) 15.5 mA cm−2, ( )
25.5 mA cm−2and ( ) 35.5 mA cm−2.
lower at the beginning of the electrolysis, achieving 69% after 90 min, and decreasing until 55% afterwards. This situation was probably due to the lower production of●OH, the enhancement of parasitic reac- tions (Eqs.12–14) as well as the production of oxidants due to the complexity of effluent, containing precursor salts.12The consumption
of electrons in non-oxidizing reactions was promoted, reducing the probability of interaction between the pollutant and●OH.
2H2O→ O2+ 4H++ 4e− [12]
2•OH→ H2O2 [13]
H2O2+•OH→ HO•2+ H2O [14]
It is important to take into account that the effluent contains an amount of Cl─dissolved, and consequently, it contributes to the elec- trochemical generation of active chlorine species. In fact, the initial concentration of chloride ions in the effluent (668 mg L−1) decreased over the treatment time, as can be observed in TableII.
Effect of addition of electrolytes into the system.—The influence of the addition of electrolytes, such as NaCl (Fig. 3a) and Na2SO4
(Fig. 3b), into the system was studied in terms of COD removal, as well as the current efficiency over time (insets of Fig. 3). Bulk experiments were performed using 1 L of the cashew-nut industry