In-situ Electrochemical Synthesis of H₂O₂ for p-nitrophenol Degradation Utilizing a Flow-through Three-dimensional Activated Carbon Cathode with Regeneration Capabilities

Abstract

The growing ubiquity of recalcitrant organic contaminants in the aqueous environment poses risks to effective and efficient water treatment and reuse. A novel three-dimensional (3D) electrochemical flow-through reactor employing activated carbon (AC) encased in a stainless-steel (SS) mesh as a cathode is proposed for the removal and degradation of a model recalcitrant contaminant p-nitrophenol (PNP), a toxic compound that is not easily biodegradable or naturally photolyzed, can accumulate and lead to adverse environmental health outcomes, and is one of the more frequently detected pollutants in the environment. As a stable 3D electrode, granular AC supported by a SS mesh frame as a cathode is hypothesized to 1) electrogenerate H₂O₂ via a 2-electron oxygen reduction reaction on the AC surface, 2) initiate decomposition of this electrogenerated H₂O₂ to form hydroxyl radicals on catalytic sites of the AC surface 3) remove PNP molecules from the waste stream via adsorption, and 4) co-locate the PNP contaminant on the carbon surface to allow for oxidation by formed hydroxyl radicals. Additionally, this design is utilized to electrochemically regenerate the AC within the cathode that is significantly saturated with PNP to allow for environmentally friendly and economic reuse of this material. Under flow conditions with optimized parameters, the 3D AC electrode is nearly 20% more effective than traditional adsorption in removing PNP. 30 grams of AC within the 3D electrode can remove 100% of the PNP compound and 92% of TOC under flow. The carbon within the 3D cathode can be electrochemically regenerated in the proposed flow system and design thereby increasing the adsorptive capacity by 60%. Moreover, in combination with continuous electrochemical treatment, the total PNP removal is enhanced by 115% over adsorption. It is anticipated this platform holds great promises to eliminate analogous contaminants as well as mixtures.

Graphical Abstract

I. INTRODUCTION

Industries such as pharmaceutics, agriculture, petrochemical, manufacturing, and mining utilize a vast array of organic compounds that end up in effluent wastewater streams and can contaminate aqueous water bodies, presenting a growing concern for global water sources. There are many classes of these organic contaminants that can be found in various aqueous matrices, and emerging recalcitrant, or persistent, organic pollutants are of great concern due to their high resistivity to natural degradation and conventional physicochemical treatment [1]. Due to the ability of these organic contaminants to persist relatively unchanged in the environment after discharge and the potential for adverse ecotoxicological effects, these pollutants pose a threat to human health and the environment [2, 3]. Therefore, due to the growing ubiquity of these recalcitrant organic contaminants in water sources, it is necessary to develop effective, efficient, and robust water treatment technologies.

Electrochemical advanced oxidation processes (EAOPs) are chemical treatment options that utilize redox reactions to degrade recalcitrant organic contaminants using electrical energy. EAOPs primarily seek to generate reactive oxygen species (ROS) (e.g., highly oxidative hydroxyl radicals (·OH)) which are considered one of the strongest oxidants applied in water treatment processes. The use of electrochemistry is a very promising treatment technique for recalcitrant pollutants involving indirect and direct methods of electron transfer. Electrochemical treatment is able to proceed with minimal addition of chemical reagents and has the ability to achieve highly effective degradation results with low current requirements. Direct anodic oxidation has rather poor decontamination performance, but indirect anodic oxidation by physiosorbed hydroxyl radicals on ‘non-active’ electrodes and chemisorbed hydroxyl radicals on ‘active’ electrodes [4] are promising for a wide range of recalcitrant pollutants [5, 6]. As well, electrochemical reduction of persistent contaminants is achievable utilizing a wide variety of cathode materials and reactor designs [7, 8]. Electrochemical methods are additionally utilized to generate chemical reagents in-situ and enhance the effectiveness of other optimized processes. The electro-Fenton technique is a very effective process in which H₂O₂ is electrogenerated via cathodic reduction of oxygen in an acidic electrolyte [9]. This process can be further enhanced by the use of high surface area carbon electrodes which increase the yield of electrogenerated H₂O₂ [10] and heterogeneous catalysts which remove the concern of iron precipitation with homogeneous ferrous ions [11]. Granular activated carbon (AC) has been utilized as a support for catalytic materials, as well as itself containing surface sites that can promote the decomposition of H₂O₂ to produce hydroxyl radicals [12]. Carbonaceous materials, including granular AC, have been utilized as electrodes because of their cost-effectiveness and high surface area in three-dimensional (3D) electrochemical regimes, though as anodes these materials have low durability and are prone to corrosion [13]. As cathodes, these 3D electrodes have the ability to electrogenerate H₂O₂ in-situ [14] and have great potential in a variety of designs. However, as ACs are still highly effective adsorbents, they can become saturated by contaminants and require disposal/replacement which is cost-prohibitive [15]. Regeneration of exhausted granular AC is feasible thermally [16], chemically, and biologically [17], though these can be energy intensive and damaging to the carbon material. Electrochemical regeneration of ACs have been the subject of recent research [18], with a focus on mechanisms, reactor design, and operating conditions for ACs saturated with contaminants such as phenol [19], toluene [20], and halogenated disinfection byproducts [21].

In the present work, a 3D electrochemical flow-through reactor employing activated carbon encased in a stainless-steel mesh as a cathode was utilized for the removal and degradation of p-nitrophenol (PNP). PNP is a highly toxic and environmental persistent organic pollutant (POP) that is not easily biodegradable or naturally photolyzed, can accumulate and lead to adverse environmental health outcomes, is one of the more frequently detected pollutants as a precursor and byproduct of pesticide use, and is here utilized as a typical representative POP [22]. Additionally, this design is utilized to electrochemically regenerate the AC within the cathode that is significantly saturated with PNP to allow for environmentally friendly and economic reuse of this material. Mechanisms and optimized operating conditions for electroperoxidation, PNP removal by adsorption/oxidation, and electrochemical regeneration of exhausted granular AC were conducted in a batch reactor (available in the Supplemental). These insights were then utilized to design and test a continuous 3D electrochemical flow-through reactor for enhanced removal/degradation of PNP, as well as electrochemical regeneration of the AC in the same designed reactor. The approach involves the optimization of reactor parameters and operating conditions to include reactor design, cathode size, AC mass/volume, applied current, flowrate, influent pH, PNP concentration, and operating time for optimal PNP removal over simple adsorption onto the AC surface. In doing so, the goal of designing a novel EAOP utilizing cheap and environmentally friendly AC as a cathode material is realized to provide efficient, robust, and maintainable treatment of aqueous recalcitrant organic contaminants.

II. EXPERIMENTAL

Experiments were conducted in both a batch and flow setting. Batch experiments were conducted to determine the effectiveness of H₂O₂ electrogeneration, PNP removal, and electrochemical regeneration of AC with the ACSS mesh cathode. Flow-through experiments were conducted in two separately sized plug flow reactors. Various parameters, such as cathode size/length, AC mass/volume, applied current, flowrate, influent pH, PNP concentration, and operating time were optimized for the electrochemical removal of PNP and regeneration of AC within the ACSS mesh cathode. A full description of utilized materials, reactor design, experimental methods, and analytical techniques is provided in the supplemental. Table 1 below shows the parameters tested and optimized for the different electrochemical reactors. Additionally, the supplemental shows diagrams and pictures of the various experiment designs, as well as tables showing the batch and flow regeneration experimental plan in more detail.

Table 1.

Experimental parameters for 1) batch H₂O₂ electrogeneration, PNP removal, AC regeneration and, 2) flow PNP removal and AC regeneration. (‘Ads’ refers to trials conducted with AC adsorption)

	Experimental Parameters
Experiment Design	AC Mass (g)	Applied Current (mA)	Na2SO4 Electrolyte (mM)	PNP Conc. (mg/L)	Flowrate (mL/ min)	Influent pH	Operation Time (min)
Batch H2O2 Electrogeneration	1	100	5	---	---	7	60
	3	75, 100, 125				3, 7, 10
	4.5	100				7
Batch PNP Removal	1	Ads, 50, 100, 150, 200	5	30	---	7	60
	3	Ads, 100
Batch AC Regeneration	1	100	50	1,000	---	7	180, 360
		300
Large Diameter Flow-Through Reactor	10	100	5	30	2	7	300
	20	Ads, 150, 200, 250, 300					180, 300, 360
	30	300					300
Small Diameter Flow-Through Reactor	10	Ads, 100	5	30	2, 5, 10	3, 7, 10	120
	20	Ads, 200			2	7
	30	Ads, 300			2	7
Flow-Through AC Regeneration	3	100	50	1,000	2	7	180, 360
		300

III. RESULTS AND DISCUSSION

3.1. Removal of p-nitrophenol Utilizing 3D ACSS Mesh Cathode Under Flow

The electrogeneration of H₂O₂ utilizing 3D carbonaceous cathodes is well known, while the use of AC for electroperoxidation is a variation on the typical utilization of reticulated vitreous carbon, carbon felts, and activated carbon fibers [23]. The fundamental reaction associated with the electrogeneration of H₂O₂ is a 2-electron oxygen reduction reaction (2e-ORR) shown in equation 1.

$${\mathit{O}}_2+2{\mathit{H}}^{+}+2{\mathit{e}}^{-}\to {\mathit{H}}_2{\mathit{O}}_2$$ Eq. 1

The in-situ generation of H₂O₂ on a conductive surface providing electrons in the presence of oxygen and an acidic electrolyte is a key factor for the performance effectiveness of many 3D electro-Fenton systems. The flow of electrons is provided by the use of the ACSS mesh electrode as a cathode, and the design of the mesh in which the AC is placed is key to ensure proper conductivity throughout the carbon material. The critical factor of electroperoxidation optimization techniques is to understand that there are additional reactions that can occur at the cathode surface that limit the electrogeneration of H₂O₂. One of these is a 4-electron oxygen reduction reaction (4e-ORR) shown in equation 2.

$${\mathit{O}}_2+4{\mathit{H}}^{+}+4{\mathit{e}}^{-}\to 2{\mathit{H}}_2\mathit{O}$$ Eq. 2

The applied current density applies to the electrons available per unit area of the activated carbon surface. At higher current densities, equation 2 may tend to dominate over equation 1 and reduce the overall electrogeneration of H₂O₂ and PNP removal. This phenomenon has been shown in designs aimed at electroperoxidation with other carbonaceous cathodes [24].

The primary removal of PNP in the novel electrochemical cell is possibly due to three mechanisms occurring. (i) The adsorption of PNP onto the surface of the activated carbon. AC adsorption has been established as an effective way for the removal of PNP from aqueous waste streams to the solid phase on the carbon surface [25]. (ii) The indirect oxidation of PNP on the Ti/MMO anode surface, which has been shown to occur on similar ‘active’ electrode materials [26]. (iii) The indirect formation of hydroxyl radicals which could possibly oxidize PNP at the activated carbon surface caused by the decomposition of electrogenerated H₂O₂ on the AC. Equations 3 and 4 below show the proposed reactions involved with this mechanism, with ‘organic contaminants’ represented by PNP in this present work [18].

$${\mathit{H}}_2{\mathit{O}}_2+\mathit{GAC}\mathit{Surface}\to 2\mathit{O}\mathit{H}\cdot$$ Eq. 3

$$\mathit{O}\mathit{H}\cdot +\mathit{organic}\mathit{contaminants}\to \mathit{degraded}\mathit{contaminants}$$ Eq. 4

The mechanisms shown by the reactions in equations 3 and 4 differ greatly from traditional catalytic mechanisms associated with electro-Fenton systems, though these have been shown effectively in similar 3D activated carbon electrochemical systems to produce hydroxyl radicals and degrade contaminants [23, 27].

The treatment of wastewaters, especially those burdened with trace contaminants at very low concentrations, require setups and designs that incorporate this increased volume of water. Plug flow reactors are more preferred due to their implementation in remedial operations, high flow industrial wastewater, and especially in point-of-entry (POE) or point-of-use (POU) water treatment systems. For the present electrochemical setup, applied current density, influent pH, flow rate, and cross-sectional area all must be considered for optimization purposes. Additionally, the AC mass not only correlates to the surface area of AC available for adsorption, electroperoxidation, and catalysis, but also to the volume/length of the cathode that contacts the PNP-laden waste stream.

Figure 1 shows the results of initial tests conducted with the large plug flow reactor for the degradation of dissolved PNP. Part A of figure 1 corresponds to a 6-hour PNP removal flow test utilizing 20 grams of AC with 200 mA of applied current (10 mA/g AC current density) compared to the same test conducted with no current applied (adsorption only). Electrochemical PNP removal is nearly 18% more effective than adsorption, with adsorption reaching steady state removal at 60% and the ACSS mesh cathode steady state removal at 78% removal. On a per mass basis shown in part B of figure 1, the ACSS mesh cathode was able to remove an average of 18.5% more PNP per gram of AC. The benefits of this electrochemical system are twofold, as this setup not only allows for enhanced removal of the parent PNP compound from the waste stream, but also induces degradation of the PNP that otherwise would be adsorbed and saturate the adsorptive surface sites of the AC.

Figure 1.

Flow removal of 30 mg/L PNP utilizing 5 mM Na₂SO₄ electrolyte and the large plug flow reactor detailed by A) comparing adsorption and ACSS mesh cathode mechanisms for PNP removal, (B) mg PNP removed per gram of AC from the C_t/C_o data in part A, (C) varying applied current density with 20g of AC mass within ACSS mesh for PNP removal, (D) mg PNP removed per ampere of electrical power for each timestep associated with part C, (E) varying mass of AC within the ACSS mesh cathode with a current density of 10 mA/g AC, and (F) mg PNP removed per gram of AC of applied current for each timestep associated with part F

Part C of figure 1 shows the results of utilizing 20g of AC within the ACSS mesh cathode with varying applied current densities of 7.5, 10, 12.5, and 15 mA/g AC. The 10 mA/g AC current density was able to achieve a 75% removal rate for this test, with drops in removal efficiency at current densities above and below this value. Lower current densities may not be sufficient to initiate 2e-ORRs and produce H₂O₂, and higher current densities may produce additional 4e-ORRs shown in equation 2 that fails to synthesize H₂O₂. However, one of the limitations with this and similar designs is the possibility for bubble oversaturation and electrowetting. Bubble evolution and accumulation at the surfaces of electrodes can reduce the surface sites of the electrode that are active for redox reactions, increase ohmic resistance due to the separation of electrodes from the electrolyte, and possibly form adverse gradients [28]. In the design of this reactor, bubbles that form from OERs at the Ti/MMO anode set below the cathode will rise due to buoyancy to contact the surface of the bottom SS mesh. At higher currents, more O₂ will form and nucleate from the anode surface, rising to the bottom of the cathode surface and reduce current active electrode sites. This has been shown in previous research [29] aimed at improving various electrochemical setups, with the main limitation of causing sharp drops in cell voltages [30]. In this setup there are sufficient spaces in the mesh to allow for diffusion of O₂ bubbles through the SS disk and into the interior of the packed AC bed at low applied currents. Previous research reveals higher currents increase bubble formation, leading to bubble oversaturation which is typical of hydrophobic cathodes with high liquid contact angles between bubbles and electrode surfaces [31,32]. Ensuring bubble oversaturation does not occur with the present cathode is key to provide the required slow diffusion of O₂ gas through the AC bed to produce 2e-ORRs while also limiting ohmic spikes. Part D of figure 1 shows the high mass removal rate of the 10 mA/g AC current density per applied energy. For each timestep the 10 mA/g current density was able to achieve over 3x more energy efficiency than other current densities.

Part E and F of figure 1 shows the results of adjusting the mass of catalyst contained within the ACSS mesh cathode and using 10, 20, and 30 grams of activated carbon (maintaining a current density of 10 mA/g AC). The 20g and 30g ACSS mesh cathodes lead to steady-state PNP removal efficiencies of 78% and 85%, respectively, while the 10g ACSS mesh cathode maintained a removal efficiency of 50%. Part F reveals that although 10g was only able to remove 50% of the PNP-laden wastewater, it was more effective at PNP removal per unit mass. The mass transfer of bulk H₂O₂ has been shown to be important for the effective decomposition on the AC surface [33], and may be reduced in the large diameter reactor. A smaller plug flow reactor was developed to lengthen the bed length of the activated carbon and increase the PNP removal efficiency with the same mass of AC. Table 2 below shows the ultimate removal efficiency results of the large and small diameter reactors utilizing varying PNP removal techniques (e.g., using an electrochemical ACSS mesh cathode or adsorption alone) current densities, flowrates, and influent pH values.

Table 2.

Removal efficiency results of the large and small diameter reactors utilizing varying PNP removal techniques (separating electrochemical PNP removal by cathodes of different AC mass values or adsorption alone), current densities, flowrates, and influent pH values

Flow Reactor	PNP Removal	Current Density (mA/g AC)	Flowrate (mL/min)	Influent pH	Removal Efficiency (%)
Large Diameter Flow-through Reactor	10g ACSS Cathode	10	2	7	50
	20g Adsorption	---	2	7	60
	20g ACSS Cathode	7.5	2	7	58
	20g ACSS Cathode	10	2	7	78
	20g ACSS Cathode	12.5	2	7	63
	20g ACSS Cathode	15	2	7	57
	30g ACSS Cathode	10	2	7	85
Small Diameter Flow-through Reactor	10g Adsorption	---	2	7	65
	10g Adsorption	---	5	7	50
	10g Adsorption	---	10	7	31
	10g ACSS Cathode	10	2	3	79
	10g ACSS Cathode	10	2	7	81
	10g ACSS Cathode	10	2	10	77
	10g ACSS Cathode	10	5	7	65
	10g ACSS Cathode	10	10	7	42
	20g Adsorption	---	2	7	81
	20g ACSS Cathode	10	2	7	91
	30g Adsorption	---	2	7	92
	30g ACSS Cathode	10	2	7	100

Figure 2 shows the results of 30 mg/L PNP removal tests utilizing the smaller plug flow reactor. Part A reflects tests conducted with increasing mass of AC within the ACSS mesh cathode, from 10, 20, and 30 grams of AC including tests utilizing adsorption only. The 10g ACSS cathode in this setup was able to effectively treat the PNP wastewater with an 80% removal efficiency, minimizing preferential flow paths seen in other activated carbon beds [34]. Additionally, with the smaller reactor, the 20g and 30g ACSS cathode depth was able to be more homogenized across the reactor surface area and lead to 91% and 100% PNP removal efficiency, respectively. The differences between electrochemical removal and adsorption for the different sized cathodes may be the enhanced current distribution throughout the entire bed length of the 10g ACSS cathode allowing better current distribution to the AC granules nearer the interior of the cathode [14]. It has been found that varying area, mass, and precursor carbon materials of different microporosities can affect the capacitance of activated carbon electrodes [35]. Additionally, the complex manner in which fluid diffuses through an electrode and the electrical conduction in a porous media affects ionic conduction in the electrode matrix [36]. More macroscopic carbon materials increase conductivity and improve the capacitance of particle electrodes, leading to greater charge distribution throughout the fixed bed [37]. A possibility is the current density is not fixed throughout the length of the bed, with higher densities forming nearer the volume located closer to the SS disks, facilitating additional 4e-ORRs (Eq. 2) and reducing the effective potential for electrogeneration of H₂O₂ and decomposition to hydroxyl radicals. This has been found in previous research to be the case above a certain current density threshold [38], though in the present study the unique design and structure of the ACSS mesh cathode presents novel insights into the current density across an AC bed for the electrochemical degradation of pollutants. The 10g ACSS mesh cathode was utilized in subsequent PNP removal tests as the optimal cathode size for enhanced removal of PNP wastewater over simple adsorption.

Figure 2.

Flow removal of 30 mg/L PNP utilizing 5 mM Na₂SO₄ electrolyte and the small plug flow reactor detailed by A) comparing adsorption and ACSS mesh cathode mechanisms for PNP removal varying AC mass with a flowrate of 2 mL/min, (B) 10 run iterations of 10g ACSS mesh cathode longevity versus adsorption for 2-hr PNP removal at 10 mA/g current density and 2 mL/min (C) varying flowrate with 10g ACSS mesh cathode at 10 mA/g current density for PNP removal, (D) mg PNP removed per gram of AC for each flowrate determination in part C, (E) varying influent pH value with 10g ACSS mesh cathode with a current density of 10 mA/g AC and 2 mL/min, and (F) 2-hr TOC removal data for experiments represented in parts A and C

Part B of figure 2 shows 10 consecutive tests utilizing the same ACSS mesh cathode utilizing a current density of 10 mA/g AC compared to adsorption with the same packed bed length of AC. Each subsequent iterative test shows the continued effectiveness of this ACSS mesh cathode over adsorption. After 10 consecutive tests, the 10g ACSS mesh cathode dropped 9% in removal efficiency while the adsorption column dropped over 17%. It is unlikely that saturation of the AC surface is a driving factor in this efficiency drop, and more due to the creation of greater preferential flow paths through the packed bed [34]. Low pressure of activated carbon in other industries reduce the formation of preferential flow paths [39], though in the present setup the compaction of AC within the packed bed is desirable for electrical conductivity. The drop in removal efficiency with the electrochemical ACSS mesh cathode was reduced compared to that of adsorption, showing the enhanced efficacy of this system for long-term and iterative water treatment. Part C and D of figure 2 shows the results of increasing flow rate on the PNP removal using the 10g ACSS mesh cathode and by adsorption. The variations between the flow rate tests show there is an effective flow rate to which the added electrochemical removal is maximized compared to simple adsorption of PNP. Part D shows that as the flow rate increases the mass of PNP removed per gram of AC also increased. These results reveal that larger flow rate scaling is possible with manipulation of plug flow designs. One of the biggest factors with flow reactors in electrochemistry is the use of these solid phase electrodes and the heterogeneous process of treating liquid phase pollutants with complex mass transfer effects [40]. It has been proposed that smaller reactor channels cause quicker electrochemical reactions and faster flow rates can elicit enhanced productivity [41], and may reveal the cause of the enhanced removal amounts per gram of AC at higher flow rates.

Part E of figure 2 shows the result of utilizing the 10g ACSS mesh cathode with a flowrate of 2 mL/min with varying influent pH values. The steady state removal of PNP after 1 hour with varied influent pH values are all nearly identical. This shows the ability of this reactor to self-regulate the internal pH of the system. The anode produces an acidic anolyte that flows up to the cathode and produces 2e-ORRs and subsequent H₂O₂ decomposition. Previous research has shown that a very similar configuration is able to produce an acidic front after the anode regardless of the influent pH value [24], revealing the cell configuration is a key factor in allowing for wide variation in influent wastewater pH. The effluent pH of the 10g ACSS mesh cathode at 2 mL/min was approximately 10.3. This high basic value follows with typical effluents following reductive cathodes in electrochemical reactors [42] and indicates there is effective electroperoxidation and indirect oxidation occurring from the acidic electrolyte formed from the anode.

Part F of figure 2 shows the TOC removal achieved after 2 hours of runtime for the tests varying AC mass within the ACSS mesh cathode as well as varying the flowrates. The 10g ACSS cathode with a 2 mL/min flowrate has 60% TOC removal, showing that there is additional breakthrough of organic compounds in the effluent. Hydroquinone and benzoquinone are two secondary compounds that can form from the hydroxyl radical induced oxidation of PNP with additional oxidation to cause ring cleavage and mineralization [43]. For the 10g ACSS cathode at 2 mL/min, 50% of the TOC is accounted for by the PNP effluent while the other half represents byproducts of hydroxyl radical oxidation occurring within the cathode. The 30g ACSS cathode removes 100% of the parent PNP compound, and the ~7% TOC breakthrough in the effluent is entirely comprised of secondary byproducts. Increasing flowrates result in reduced TOC removal efficiency. These insights reveal the power of these cathode configurations for not only TOC removal, but also the efficacy of oxidation with the varying configurations. Understanding the effluent characteristics of single plug flow reactors can drive more practical implementation of these type of reactors either into plug flow systems in series, parallel, or an optimized combination for the most efficient mass and flow utilization.

3.2. Electrochemical Regeneration of PNP-Saturated GAC Under Flow

The novelty of the presented system lies not only in the use of cheap and abundant activated carbon as a cathode in a flow setting for the removal of PNP wastewater, but the manner in which this system can be operated to allow for the extended use of activated carbon as an adsorbent. Various methods of regenerating ACs have been tried using many techniques, and electrochemical regeneration is very promising but has little investigation into its applicability for large-scale systems.

A balance between regeneration efficiency and energy consumption is essential to seek an economical method for AC regeneration utilizing electrochemistry. Equation 5 details the specific energy consumption, E_sp (kWh/g), per amount of pollutant removed.

$${\mathit{E}}_{\mathit{sp}}=\frac{\mathit{E}\mathit{x}\mathit{I}\mathit{x}\mathit{t}}{\mathit{V}\mathit{x}\Delta \mathit{R}}$$ Eq. 5

Where E is the average cell voltage, I is the applied current, t is the full time of electrical input, V is the total volume treated, and ΔR is the total change in pollutant concentration due to adsorption. This metric combines the electrical input with the total removal by adsorption to normalize the efficiency of electricity in regenerating this AC. It has been generally accepted that longer treatment times lead to better regeneration capabilities [23, 44] to a certain extent. Longer treatment times can lead to more sequential oxidation reactions to take place, though there is the obvious limitation of increased energy consumption and possible altering of the carbon surface [45].

A vast majority of GAC regeneration techniques are performed in batch reactor setups with no continual influent or effluent streams. This allows for careful management of internal environmental conditions, ease of reactor design, and potentially more regeneration efficiency, though there are practical and operational limitations. AC is primarily used in industry in continuous flow operations, allowing for constant sequestration of aqueous pollutants from the liquid phase to the solid phase. In order to electrochemically regenerate activated carbon in methods and techniques previously researched [18], the granules are placed in batch-type reactors where regeneration of adsorptive capacities of ACs can be achieved. This, however, may require handling and transportation of contaminated GAC from their use in packed flow-through beds to these batch-type reactors which can be costly, hazardous, and operationally challenging. Therefore, it is necessary to design electrochemical regeneration processes that utilize flow designs to better represent the typical implementation of AC in industrial water/wastewater treatment.

A crucial novel concept is the development of ‘trap-n-zap’ electrodes which perform two key functions which are represented in the present work. The main factors associated with real-world wastewater streams (excluding industrial effluents) are dilute concentrations of trace pollutants in water bodies resulting in mass transfer limitations. These low pollutant concentrations severely drop the observed rate values for electrochemical treatment due to low electrode surface concentrations. The first ‘trap’ step in ‘trap-n-zap’ is the selective attraction of pollutants in a waste stream to electrode surfaces through adsorption or molecular sieving [46, 47] which is able to overcome mass transfer concerns and increase the pollutant concentration on the electrode surface. Additionally, this increases the selectivity of the subsequent treatment for the target pollutant and not of other scavenging intermediates present within wastewater streams. The ‘zap’ step consists of utilizing electrochemical redox processes for the direct/indirect degradation of ‘trapped’ pollutants [48]. This type of ‘trap-n-zap’ process is made more effective in a flow regime that can handle extremely high volumes of wastewater with very low concentrations of target pollutants, with ‘trapping’ materials such as AC that are non-selective and highly robust. In the present electrochemical setup, the AC granules within the SS mesh cathode act as ‘trapping’ materials for sequestering a large mass of the PNP contaminant (simulating the removal of trace pollutants within a continuous flowing waste stream) within the pore structure and upon the AC surface. Electrochemistry is then utilized as the ‘zapping’ technique to initiate electrochemical redox processes for the direct/indirect degradation and removal of PNP from the AC material and extend its usable life as a ‘trapping’ material.

Part A of figure 3 shows the results of this present work’s small plug reactor design at regenerating highly saturated AC in a flow regime. AC has the ability to adsorb large masses of PNP from waste streams, and increasing the concentration of influent PNP to 1000 mg/L allowed for partial saturation of the surface and pore structure of the AC within the ACSS mesh. The ‘No Regeneration’ setup provided a baseline essential to show the effectiveness of the subsequent electrochemical flow regeneration setups in increasing the adsorptive capacity of the AC. 6-hr regeneration at 100 mA is >10% more effective for each trial than the 3-hr regeneration. Conversely, the 3-hr regeneration at 300 mA setup is >15% more effective for each trial than the 6-hr regeneration. The 6-hr regeneration at 100 mA was the most effective at regenerating the AC and more economically viable than utilizing higher currents with lower regeneration times.

Figure 3.

Plug-flow regeneration of AC contained with the ACSS mesh utilizing 3 gram AC and varying applied currents/regeneration times to include regeneration of previously utilized ACSS mesh cathodes (applied current densities of 10 mA/g) that have become saturated. A flowrate of 2 mL/min was utilized for all trials

The adsorption flow setups were compared to the use of 3g of AC with an optimal current density of 10 mA/g AC utilized within the ACSS mesh cathode for in-situ electrochemical treatment of the 1000 mg/L PNP waste stream instead of simply utilizing adsorption. In electrochemical treatments, increased pollutant concentration results in faster reaction rates and greater overall contaminant removal over simple adsorption [49]. Comparing the total mass of PNP removed from all 5 trials and the E_sp values reveal further insights into the advantages of these electrochemical regeneration setups over adsorption. Comparing the setups in table 3, the 10 mA/g AC ACSS cathode with no regeneration has only slightly lower overall removal than several adsorption tests with iterative electrochemical regeneration. This reveals both the effectiveness of the regeneration steps and the operational effectiveness of the ACSS mesh cathode with no regeneration. However, the 10 mA/g ACSS cathode with an additional regeneration step was superior in total mass of PNP removed and had a relatively low E_sp value. The in-situ electrochemical treatment of PNP-laden waste streams is superior to adsorption, and additional regeneration of this ACSS mesh cathode without having to manipulate the design or handle the contaminated AC makes this setup operationally superior over batch regeneration. The implementation of the highly concentrated electrolyte for regeneration is a consideration for design as well, similar to a filter backwash.

Table 3.

E_sp Values for Novel Electrochemical Regeneration of PNP Under Flow

Configuration	Time of Applied Amperage (hr)	Applied Amperage (A)	Mass PNP Removed (g)	Esp (kWh/g PNP)
No Regeneration	0	0	0.48	0
100 mA, 3-hr	12	0.1	0.67	.0015
100 mA, 6-hr	24	0.1	0.77	.0026
300 mA, 3-hr	12	0.3	0.72	0.0083
300 mA, 6-hr	24	0.3	0.58	0.0207
10 mA/g AC Cathode, No Regeneration	30	0.03	0.68	0.0011
10 mA/g AC Cathode, 100 mA/3-hr Regeneration	42	0.061	1.03	0.0027

One of the main factors with electrochemical regeneration of carbons is the electrolyte concentration. 50 mM Na₂SO₄ was the utilized electrolyte for use in regenerating the AC in these setups, and increased electrolyte concentration has been shown to increase regeneration efficiency due to increased current flow and oxidation rates [27]. Additionally, based on equation 5, an increase in electrolyte concentration decreases the ohmic resistance of the water column and drops the cell voltage as constant current is applied, making the setup more energy efficient (although with more chemical additives). NaCl electrolytes are typically used but can lead to the chlorination of organic compounds [45,50]. Sodium sulfate has shown to elicit lower regeneration efficiencies [51], however not forming chlorinated byproducts is advantageous in a system that seeks to produce cost-effective and efficient results. With the unique use of regeneration within the flow regime, degradation byproducts that may be formed from oxidation of ‘trapped’ PNP on the AC surface and become detached from the cathode do not remain in solution. The movement away from the cathode surface allows for the enhanced efficiency of this type of system over batch configurations. For design considerations, a recycle of strong electrolyte can be utilized so that the effluent of the regeneration column is returned back to the influent and any degradation byproducts that may be formed and enter the effluent can be further treated during the regeneration/electrochemical treatment step. These mechanisms would be highly complex and vary for any given waste stream, however this setup could further increase the treatment effectiveness utilizing the ACSS mesh cathode.

Another important consideration is the potential change in the point of zero charge pH (pH_pzc) due to this setup. Primarily the concern is the electrochemical treatment of AC may affect the surface chemistry of the AC and drastically change the pH_pzc, reducing the attraction between the surface and PNP. The surface charge of the catalyst (AC in this case) combined with the protonated/deprotonated configuration of the pollutant leads to varying degrees of attraction and repulsion that affect the proximity to which the pollutant can reach the surface. At the catalyst surface, where H₂O₂ can form and decompose to hydroxyl radicals with short Nernst diffusion distances, the more attractive a surface is to the pollutant, the better the chance for oxidation. Typically, a catalyst will have a surface charge of zero in a particular solution pH where below this value it will adsorb H⁺ ions and become more positively charged and above this will adsorb more OH⁻ and be more negatively charged [12]. Typical unmodified GAC pH_pzc is around 10.2-10.3 [52], and in the present work found to be 10.15. PNP has a pKa value of 7.15, and there are beneficial electrostatic adsorption interactions occurring between its cationic form and the AC surface. The pH_pzc of the AC after electrochemical treatment was determined to be 9.75. The electrochemical treatment did not drop the pH_pzc value below the pKa of PNP or significantly closer to this value, revealing that attractive surface forces are sustained and oxidation by formed hydroxyl radicals can proceed for continued AC regeneration.

These regeneration efficiencies are representative of this specific system, parameters, and target pollutant, but reveals how a ‘trap-n-zap’ solution can be formed using an 3D AC cathode in flow conditions. Additionally, the unique design and implementation of utilizing the 3D ACSS mesh cathode for in-situ electrochemical treatment as well as for electrochemical regeneration is vital for the long-term use of GAC. This key point has been made by previous research [51] in that while certain indirect anodic oxidation mechanisms are effective for regenerating carbons polluted with specific contaminants, cathodic polarized GAC is more favorable due to the decrease in surface oxygen content and for extending the usable life of the carbon surface. Ultimately, using AC within a SS mesh for the continuous electrochemical treatment of PNP and regenerating the carbon surface with a strong electrolyte once it has been saturated is key to achieving a long-term and economically viable water treatment system for recalcitrant organic contaminants.

IV. CONCLUSIONS

The objective of this work was to implement an AC-based catalyst as an electrode to act as both a cathode and catalyst for the simultaneous electrogeneration of H₂O₂ and decomposition to hydroxyl radicals to treat a recalcitrant contaminant p-nitrophenol. Due to the fact that AC itself can be a three-dimensional cathode material to reduce oxygen in an acidic electrolyte to form H₂O₂ and has a surface chemistry with catalytic abilities to decompose H₂O₂ to hydroxyl radicals, implementing this novel catalytic cathode in a flow-regime with a wastewater influent can create opportunities for robust, maintainable, and operationally viable treatment designs. The AC cathode was implemented in a flow regime with two different sized plug flow reactors to effectively remove PNP with a focus on oxidation over adsorption, complete removal, efficiency of the system considering mass of contaminant removed per mass of AC used. As well, the variations to PNP removal effectiveness with differing catalyst masses, applied current density, influent pH, and flowrate were considered. Finally, to represent the strength of this design for real-world applications and the power of electrochemistry for efficient water treatment and reuse technologies, the electrochemical regeneration of the exhausted AC catalyst within the cathode was examined. In a novel research strategy to determine the electrochemical regeneration of AC within a flow regime, the applied current, regeneration time, and the regeneration of AC utilized in electrochemical oxidation experiments were evaluated in the plug flow-through reactor for the continuous treatment and reuse of the AC material.

The following main conclusions can be drawn from the presented work:

• The ACSS mesh cathode is far superior after continued use than either adsorption, anodic oxidation, or both combined for removal of PNP. Under flow the ACSS mesh cathode was 18% more effective in removing PNP from solution than adsorption alone. A plug flow reactor with a smaller diameter and longer bed lengths led to 80%, 91%, and 100% steady state removal of PNP with the 10, 20, and 30g ACSS mesh cathodes, respectively.

• Longevity tests revealed that the ACSS mesh cathode was more effective over a longer time period and had a less significant drop in effectiveness than an adsorption column. Increasing flowrate reduced the total PNP removal % but significantly increased the mass of PNP removed per gram of AC utilized. The influent pH of the system had little effect on the steady state removal effectiveness of the plug flow reactor using the ACSS mesh cathode.

• The AC within the SS mesh cathode is able to be electrochemically regenerated in the present flow system and design. This technique represents one of the first of its kind in ‘trap-n-zap’ technique of simulating the accumulation of trace pollutants on an electrode surface and using electrochemistry to treat the heavily concentrated pollutant to overcome mass transfer limitations. Optimized flow regeneration parameters increased the adsorptive capacity of the AC by 60% more than AC that had not been regenerated. An ACSS mesh cathode with optimized regeneration steps was able to remove nearly 115% more PNP than AC that had not been regenerated.

• This design represents an electrochemical treatment process with a unique 3D activated carbon electrode with heterogeneous catalytic abilities that can overcome mass transfer limitations with trace pollutants, effectively treat persistent aqueous pollutants continuously, and extend the usable lifetime of this carbon material and system.