Silica Adsorbents and Peroxide Functionality for Removing Paraquat from Wastewater Downloaded from ascelibrary.org by New York University on 05/17/15 Copyright ASCE For personal use only; all rights reserved Shama F Barna 1; Elizabeth A Ott 2; Thu H Nguyen 3; Mark A Shannon 4; and Alexander Scheeline Abstract: To treat wastewater containing paraquat, the interaction of the herbicide with unmodified and modified (i.e., chemically treated) proprietary silicas is explored using liquid chromatography—mass spectrometry and ultraviolet-visible spectrometry Strong adsorption and rapid exchange kinetics (i.e., more than 80% within 30 s) of the contaminant onto both unmodified and modified silica surfaces indicate that paraquat adsorption is inherent to the unmodified silica Although activated carbon sequesters paraquat more effectively than silica at long times (i.e., after h), considering both constant mass and constant area approaches in loading the adsorbents, the best performance in the short-term (i.e., at least up to 150 s) interaction was achieved by using unmodified silicas Comparison of data fitting to a competitive binding model with or without consideration of homogeneous solid diffusion indicates that, at least for times relevant to water-treatment applications, the competitive binding model is an appropriate mathematical tool to explain experimental adsorption data Addition of H2 O2 is also attempted to initiate paraquat decomposition; however, silica is not an effective catalyst for peroxide decomposition paraquat DOI: 10.1061/ (ASCE)EE.1943-7870.0000702 © 2013 American Society of Civil Engineers CE Database subject headings: Water treatment; Adsorption; Wastewater management; Abatement and removal; Kinetics Author keywords: Water treatment; Paraquat removal; Silica adsorbents; Peroxide functionality Introduction Paraquat dichloride, as an acutely toxic herbicide for agricultural application, has been a major concern in recent years owing to the associated risk of accidental or intentional contamination of water with the compound (Clark et al 1966; Simon and Taylor 1989; Sittipunt 2005) Although various methods, such as chemical/photocatalytic oxidation, coagulation, and biological degradation, for removal of pesticides and various dyes from water are reported in the literature (Churchley 1994; Stephenson and Duff 1996; Salem and El-Maazawi 2000), techniques to utilize these methods in an economical fashion are still continuing topics of research Photocatalytic treatment of water is not yet an energyefficient solution to the problem because photocatalysts that are Graduate Research Assistant, Dept of Mechanical Science and Engineering, Univ of Illinois at Urbana-Champaign, 1206 W Green St., Urbana, IL 61801 E-mail: barna1@illinois.edu Chemist I, Abbott Laboratories, 100 Abbott Park Rd., Abbott Park, IL 60064; formerly, Undergraduate Research Assistant, Dept of Chemistry, Univ of Illinois, 600 S Mathews Ave 61, Urbana, IL 61801 E-mail: eaott2@gmail.com Undergraduate Student, Hanoi Univ of Science—VNU, 19 Le Thanh Tong St., Hanoi 10000, Vietnam; formerly, Undergraduate Research Assistant, Dept of Chemistry, Univ of Illinois, 600 S Mathews Ave 61, Urbana, IL 61801 E-mail: hoaithu.hus@gmail.com Deceased; formerly, Professor, Dept of Mechanical Science and Engineering, Univ of Illinois at Urbana-Champaign, 1206 W Green St., Urbana, IL 61801 Professor, Dept of Chemistry, Univ of Illinois, 600 S Mathews Ave 61, Urbana, IL 61801 (corresponding author) E-mail: scheelin@illinois edu Note This manuscript was submitted on April 10, 2012; approved on January 30, 2013; published online on February 1, 2013 Discussion period open until December 1, 2013; separate discussions must be submitted for individual papers This paper is part of the Journal of Environmental Engineering, Vol 139, No 7, July 1, 2013 © ASCE, ISSN 0733-9372/2013/ 7-975-985/$25.00 stable and capable of providing sustained photocatalysis using lowcost visible lamps or sunlight are as yet unavailable, ultraviolet (UV) light-emitting diodes (LEDs) are still expensive, and chemical treatments using exogenous agents are only effective if required doses of oxidants are low Many of the contaminants (e.g., dyes) are toxic to the organisms used in biological decontamination processes, and therefore such treatments are not always applicable Consequently, for flexibility, ease of operation, and insensitivity to toxic pollutants, adsorption has been a key technology (Ali and Gupta 2007) for direct removal of chemical pollutants from water and for facilitating destruction of the contaminant by catalyzed chemical reaction Although use of activated carbon as an adsorbent material is widespread, commercial production of the adsorbent produces toxic gases and polycyclic aromatic hydrocarbons during carbonization of the precursor material and is considered to be an energyintensive process because of the high temperatures required for thermal activation, regeneration, and reactivation of the material (ầeỗen and ệzgỹr 2011) A wide range of low-cost alternative adsorbents, including different natural minerals (e.g., clay, siliceous materials, or zeolites), agricultural waste (e.g., sawdust, orange peels, or banana peels), and industrial by-products [e.g., bagasse fly ash, iron(III) hydroxide, or red mud], have been investigated for removal of different metal contaminants and dyes from wastewater (Pollard et al 1992; Crini 2006; Gupta and Suhas 2009) Hence, while simultaneously exploring other potential mechanisms to treat paraquat-containing water (e.g., oxidation of paraquat with H2 O2 ), research to identify low-cost alternative materials capable of effectively adsorbing paraquat is needed The idea is to develop an efficient, rapidly deployable, and economic paraquat-removal system that may offer the simplicity of an adsorption-based mechanism, but to minimize the complications of chemical oxidation– based treatment (such as finding a suitable catalyst, generating oxidants, or providing energy-efficient shortwave illumination) In addition to reports of paraquat uptake onto treated and untreated activated-carbon products (Dhaouadi and Adhoum 2010), JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 / 975 J Environ Eng 2013.139:975-985 Downloaded from ascelibrary.org by New York University on 05/17/15 Copyright ASCE For personal use only; all rights reserved adsorption of the particular contaminant onto different natural minerals and treated/regenerated minerals (spent and treated diatomaceous earth, activated clay mineral, kaolinite, illite, and montmorillonite) has already been explored to determine the environmental fate of the compound or estimate the potential of the adsorbents for treating water containing paraquat (Draoui et al 1999; Tsai et al 2003, 2005; Tsai and Lai 2006) Although such natural minerals are promising as low-cost adsorbents for watertreatment applications, the primary constituents that govern the adsorption process are still undetermined, and therefore the adsorbing characteristics reported in the literature are case specific and may vary depending on the constituents and origins of the materials (Gupta et al 2009) With oxygen (46.1% by weight) and silicon (28.2% by weight) being the first and second most abundant elements on earth, respectively (Lide 1996), silica is a widely available and inexpensive material to manufacture A thermodynamic study of paraquat adsorption onto different minerals concluded that the cationic exchange capacity of silica can play a major role in the interaction of the compound with cationic organic compounds (Draoui et al 1999) Though a recent investigation reports uptake of paraquat onto surfaces of silica modified with titania (Brigante and Schulz 2011), a detailed examination of paraquat uptake onto unmodified silica with insights into the overall uptake kinetics and efficacy of the material as a water-treatment agent does not appear in the literature By utilizing ultraviolet-visible absorption spectrophotometry (henceforth, UV-Vis) and liquid chromatography—mass spectrometry (LC-MS) as analytical tools, the individual interactions of five unmodified and 11 modified proprietary silica materials with paraquat are experimentally investigated The idea is to compare the adsorption performance of the unmodified and modified materials and thus identify the intrinsic adsorption characteristics of unmodified silica The nature of the paraquat—silica interaction is characterized by modeling the experimental data with well-known mathematical adsorption equations The UV-Vis assay was repeated for commercially available activated carbon to have a basis of comparison for paraquat-removal efficiency of the silica products Knowledge obtained from the particle system can suggest the required particulate loading and configuration (e.g., serial, parallel, tubular, or spiral) of silica-coated membranes to treat water containing paraquat Additionally, the interaction between H2 O2 and dicationic paraquat in aqueous solution was also explored to estimate the effectiveness of peroxide in removing the contaminant Materials and Methods Reagents Paraquat dichloride as purchased from Sigma Aldrich Because paraquat salts are highly hygroscopic, the powder was dried at 100°C for h immediately before use to ensure that it reached constant weight before being used as a standard After drying the powder was cooled and stored in a desiccator To prepare stock paraquat solutions, the dried paraquat salt was weight using an adequately sensitive Mettler model XP26 microbalance, and then sample paraquat solutions of various concentrations were prepared by diluting aliquots of the stock solutions All silica samples used are proprietary to PPG Industries; to protect their identities, samples are referred to using a sample number throughout the article Samples 1–4 and Sample are unmodified silica, whereas the rest of the silica samples are modified (i.e., chemically treated) by PPG Industries Among the unmodified silica samples, Sample and Sample are commercially known as Hi-Sil 135 and Lo-Vel 6000 and have particle size of 10 and 16 μm, respectively (PPG Industries 2012) Activated carbon (Darco G60) was purchased from Fisher Scientific As per the information provided by the supplier, the product is steam activated, contains 100% (weight) carbon, and particle size distribution is 100–325 mesh (44–149 μm) All the samples except silica samples 13–16 were tested as received Samples 13–16 were dried at 100°C before use, as recommended by the supplier For the H2 O2 oxidation experiments, commercially available standard 0.1-M ceric sulfate [CeðSO4 Þ2 ] solution was used to standardize H2 O2 The concentration of the reagent-grade H2 O2 used was found to be 30.5% w=v Analytical Methods Spectrophotometric Assay All spectrophotometric measurements were made on a HewlettPackard (now Agilent) 8452A diode-array spectrophotometer Two different approaches are available for spectrometric determination of paraquat residues in water (Yuen et al 1967; Ashley 1970; Kuo et al 2001): Measuring UV absorption of paraquat ions in aqueous solution (because there is a greater tendency of formulation additives to interfere at the lower wavelengths, this method can only be considered reliable for very pure aqueous solutions of paraquat); and Generating paraquat radicals by alkaline dithionite reduction, followed by determination of paraquat residues from the peak in the visible absorption spectrum (this method is considered convenient for determination of paraquat in formulated products) Because the calibration for the method provided a reproducible detection limit, UV-Vis absorptiometry was chosen to be the appropriate method for this research Further, any degradation might interfere chemically or spectroscopically with the assay Because the paraquat solutions prepared in the laboratory are free of additives, the possibility of interference by any undesired compound other than degradation products is moot Calibration of the spectrophotometer for UV absorption of paraquat at 258 nm was performed at pH ¼ 8.06 (a 0.025-M borate buffer) A calibration curve for H2 O2 at the peak wavelength of paraquat absorption (258 nm) was also produced to facilitate understanding of the chemistry in H2 O2 -added paraquat solutions The second calibration was performed at pH ¼ 5.85 (in acetate buffer) so that the spectrometric results for H2 O2 -added paraquat solutions can be compared with the LC-MS results for unbuffered (pH ¼ 5.85) H2 O2 -added paraquat solutions The pH of the solutions was measured using a pH meter, and absorbance measurements of the solutions were made by the Hewlett-Packard 8542A diode-array spectrophotometer Linear regression analysis was used to construct Beer’s Law working curves Working curve linear regression equations, including 95% confidence intervals for the regression coefficients, are as follows: • For paraquat in borate buffer at pH 8.06 (ppm = parts per million): A258 nmị ẳ 0.0716 ặ 0.0015ịCparaquat ppmị ỵ 0.03 ặ 0.02ị), R2 ẳ 0.9956; and For H2 O2 A258 nmị ẳ 0.0181 ặ 0.0001ịCparaquat ppmị ỵ 0.001 ặ 0.003ị), R2 ẳ 0.9997 Liquid ChromatographyMass Spectrometry Analysis Chromatographic separation was performed using a Waters 2795 separation module (reverse-phase high-pressure LC) equipped with a quaternary solvent delivery system, autosampler, and column heater A 2.1- × 50-mm Eclipse XDB C18 column was used 976 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 J Environ Eng 2013.139:975-985 Downloaded from ascelibrary.org by New York University on 05/17/15 Copyright ASCE For personal use only; all rights reserved Before testing the samples, the column was first decontaminated from previously used solutions by running a 100-ppm unbuffered aqueous solution of paraquat through the system Gradient elution was used for separation of the aliquots by varying the polarity of the mobile phase Solvent A was a mixture of 95% water and 5% acetonitrile, and Solvent B was a mixture of 95% acetonitrile and 5% water The elution solvent used was initially 100% Solvent A Over min, the solvent mixture was gradually changed to 100% Solvent B From 9–10 minutes, the solvent mixture was rapidly returned to 100% Solvent A Mass spectrometry was carried out using a Waters Quattro Ultima mass spectrometer equipped with electrospray ionization and operated using MassLynx 4.1 MS software (Waters 2012) Ionization takes place in the source at atmospheric pressure The ions are sampled through a series of orifices into the first quadrupole, where they are filtered according to their mass-to-charge ratio (m=z) Data acquisition was performed between m=z ¼ 100 and m=z ¼ 500 Measurements of BET-N2 The BET-N2 –specific surface area measurement was performed using a Quantachrome Nova 2200e instrument The specific surface areas for N2 adsorption at 77.3 K (−195.7°C) were calculated by the instrument using the BET model in the range of 0.1 < P=Po < 0.2 (Samples 1–12) Paraquat solutions were separately mixed with the silica samples for before adding H2 O2 to the solutions The aim of this method was to saturate the active adsorption sites on the surface of the adsorbents and thus to distinguish between the contributions of adsorption and H2 O2 -assisted decomposition in removing paraquat To allow adsorption data normalization, BET-N2 analysis was conducted to determine the specific surface areas of the silicas Because there is restriction on which data can be shared on the modified materials that are proprietary to PPG Industries, all the unmodified silicas (Samples 1–4 and Sample 6), two modified silicas (Sample and Sample 10), and the activated-carbon sample were analyzed The instrument allows analyzing two samples at a time Before conducting the analysis, each of the two empty sample cells was weighed Then, samples were loaded into the cells and degassed at 300°C for h to remove volatile species from the materials The sample cell containing the sample was weighed again after degassing The idea was to determine the exact quantity of each sample used because the information is needed for specific surface area calculation by the instrument Results of the specific surface area measurements are presented in Table The BET surface area obtained by this analysis for activated carbon (Darco G60) was found to be similar to the BET measurement reported in previously published literature (Dun et al 1985; Muradov 2000) Experimentation All experiments were conducted at room temperature (25°C) For spectrometric analysis of paraquat-adsorption kinetics, each of the silica samples was mixed separately with buffered proxy (paraquat dichloride) solution The initial concentration of paraquat used in each trial was 25 ppm Each silica sample was added to the 30-mL paraquat solution to give a final silica concentration of 1.67 g=L The system was kept in suspension using a magnetic stirrer For both UV-Vis and LC-MS (discussed in the following paragraph) analyses, the solution aliquots were collected using 0.45-μm Millipore filters to filter out the silica particles The effect (e.g., loss of paraquat from sample solution) of filters on the measurements was investigated and found to be negligible For UV-Vis measurements, sample aliquots were initially collected at regular intervals of s for the first 30 s, and then at intervals of 30 s for the next 120 s To determine the saturation adsorption capacity of silica particulates, three additional data were also collected for each of the samples over 48 h The idea was to determine when the paraquat— silica interaction reaches equilibrium and thus to identify the total useful adsorption capacity using the equilibrium adsorption data The first few drops of the filtrate were used to decontaminate the cuvette from previous solutions, and the rest was used for absorbance measurement with the spectrophotometer The LC-MS analysis of the samples was conducted to investigate whether the paraquat uptake onto the silica particles is accompanied by any chemical dissociation in the presence or absence of H2 O2 To reduce noise in the mass spectra from buffer components, 100-ppm paraquat solutions were prepared using only deionized water For the LC-MS analysis, three different silica concentrations (1.3, 1.67, and g=L) were added separately to mL of the paraquat dichloride solutions, and the system was kept in suspension using a magnetic stirrer The idea behind varying the silica concentration was to investigate whether any reaction of paraquat with silica can be initiated The LC-MS assays of paraquat decomposition were conducted by adding H2 O2 to unbuffered paraquat solutions (pH ¼ 5.85) both in the presence and absence of presaturated silica particles Modeling of Adsorption Kinetics Langmuir Adsorption Model According to Langmuir, adsorbate molecules are assumed to bind to distinct empty sites on the surfaces of the adsorbent through the following reversible process (Zuyi and Taiwei 2000; Chiron et al 2003): ka A ỵ SA S kd where A = adsorbate molecule; S = adsorption site on surface of adsorbent; ka and kd = adsorption and desorption rate constants, respectively; and A − S = complex formed owing to adsorption of solute molecule onto adsorbent surface The basic idea behind the Langmuir adsorption model is the formation of a homogeneous adsorbent surface by a monomolecular layer of adsorbate molecules at isoenergetic, uncooperative sites As per this adsorption model, the fraction of the adsorbent surface occupied by the solute (θ) can be expressed as a nonlinear function of solute concentration (C) by the following equation: ẳ KcC ỵ KcC Kc ẳ ka kd 1ị Furthermore, by balancing the relative rates of uptake and release, the Langmuir adsorption isotherm rate of change of surface coverage (dθ=dt) can be modeled [Eq (2)] and then can be integrated to predict the adsorption as a function of time (Chiron et al 2003): d ẳ ka ịC kd dt 2ị Integrating Eq (2) and taking K ẳ C=C ỵ kd =ka ị and K ẳ ka C ỵ kd , the fractional surface coverage can be calculated as a function of time: 00 JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 / 977 J Environ Eng 2013.139:975-985 978 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 J Environ Eng 2013.139:975-985 150 379 610 544 500 530 — — — 267 — — — — — — 895 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 Initial paraquat concentration (ppm) 42.5 29 22.7 22.6 26.8 25.4 — — — 21.9 — — — — — — 13 Mass adsorbed per unit surface area (μg=m2 ) 6.3 11 13.8 12.3 13.4 13.4 — — — 5.8 — — — — — — 11.6 Mass adsorbed per unit mass (μg=mg) Adsorption data for 30 s 42 73 91 81 88 90 82 82 52 39 41 33 78 64 44 73 77 Paraquat adsorbed per 50 mg of adsorbent (percentage) 91.2 37.3 24.4 27.4 29.6 28.2 — — — 54 — — — — — — 16.9 Mass adsorbed per unit surface area (μg=m2 ) 13.7 14.2 14.9 14.9 14.8 14.9 — — — 14.4 — — — — — — 15.1 Mass adsorbed per unit mass (μg=mg) Adsorption data for 48 h 90 93 98 98 97 98 95 96 75 95 61 50 97 96 82 99 100 Paraquat adsorbed per 50 mg of adsorbent (percentage) Note: Adsorption data were normalized for surface area and for mass of adsorbents used by utilizing known information such as BET-N2 —specific surface area measurements, volume of 25-ppm paraquat used (30 mL), and loading of adsorbents (1.67 g=L) Silica Silica Silica Silica Silica Silica Silica Silica Silica Silica 10 Silica 11 Silica 12 Silica 13 (dried) Silica 14 (dried) Silica 15 (dried) Silica 16 (dried) Activated carbon Particle type BET-N2 –specific surface area (m2 =g) Table Paraquat-Adsorption Data at 30 s and 48 h Downloaded from ascelibrary.org by New York University on 05/17/15 Copyright ASCE For personal use only; all rights reserved ð3Þ Competitive Binding Adsorption Model Downloaded from ascelibrary.org by New York University on 05/17/15 Copyright ASCE For personal use only; all rights reserved Competitive binding assays are based on the idea that two molecules, A and B, compete for the same adsorption site, and adsorption of one molecule (A) onto an adsorption site is accompanied by the desorption of the previously adsorbed molecule (B) from that particular site (Sklar et al 1985; Bachas and Meyerhoff 1986) The adsorption process can be represented by the following interaction: ka A ỵ B SA S ỵ B kd Assuming all the surface sites are initially saturated with molecule B before adsorption of A occurs, fractional coverage of the surface by adsorption of molecule A (θA ) can be expressed as a function of the bulk solution concentration (C) by balancing the adsorption and desorption rates of A at equilibrium: θA ẳ KC ỵ KC 4ị where ẳ A ỵ B ị = fraction of surface sites occupied by adsorbates A and B at equilibrium; and K ¼ ðka =kd Þ = ratio of adsorption constant to desorption constant of A For this model, the adsorption kinetics is governed by the following equation: A tị ẳ K1t ỵ K1t 5ị Cs ẳ Ctị 6ị where Cs = liquid-phase concentration of paraquat at liquid—silica interface Homogeneous Solid Diffusion Model Governing Equation with Boundary and Initial Conditions The homogeneous solid diffusion model (HSDM) [Eq (6)] accounts for transport of the adsorbate within a spherical particle and has been used in the literature in combination with popular adsorption isotherms (i.e., Langmuir or Freundlich isotherms) to model adsorption (McKay 1998; Meshko et al 1999, 2001; Veliev et al 2006): ∂q ∂ cq ¼ Ds r2 ∂r r ∂r ∂r 00 7bị 7cị where V = volume of solution used; and M = quantity of silica used The axisymmetry stated in Eq (7b) arises from the assumption that the adsorbent is spherical (Meshko et al 1999) If adsorption at surface sites and internal mass transfer within the particle are considered, Eq (7c) can be imposed, and external mass transfer resistance in the hydrodynamic film of the particle is considered negligible The initial condition is qẳ0 0rR tẳ0 7dị Method of Computation The diffusion equation provided in Eq (7a) was discretized following the Crank-Nicholson scheme (Veliev et al 2006): qj;m qj;m1 Ds qjỵ1;m qj;m qj;m − qj−1;m ¼ rj−1 − rj t h h2 qjỵ1;m1 qj;m1 qj;m1 qj1;m1 ỵ rj − rj−1 h2 h2 1≤j≤J 1≤m≤T ð8Þ where T = total number of time steps; and J = total number of nodes in radial direction from center to surface of particle The set of equations that result from Eq (8) at different nodes (j) were solved numerically in MATLAB (MathWorks 2010) for different time steps (m) by formulating a matrix based on the tridiagonal matrix algorithm (TDMA) (Conte and deBoor 1972) Results and Discussion Adsorption Kinetics of Paraquat A significant decrease in paraquat concentration was noticed within the first s after adding any of the silicas (Figs and 2) Sample 3, an unmodified silica, outperformed any of the samples tested when percentage of paraquat-removal data for the first 30 s were considered (Table 1) Compared with the 77% paraquat adsorption onto activated carbon, more than 80% of paraquat was removed within 30 s of first paraquat–silica interaction by unmodified silica Samples (91%) and (90%) and modified Samples (88%) and (82%) For all tested samples except Samples 1, 9, 11, and 15, paraquat adsorption tends to reach equilibrium within the maximum 30 Sample-1 Sample-2 Sample-3 Sample-4 Sample-5 Sample-6 Sample-7 Sample-8 25 [paraquat] (ppm) 00 tị ẳ K eK t ị 20 15 10 0 10 15 Time (sec) 20 25 30 Fig Measured paraquat concentrations for silica Samples 1–8 up to 30 s after adding particles [sample aliquots were collected at 5-s intervals; solution pH was 8.04 in borate buffer; silica concentration in 25-ppm paraquat solution (initial concentration) was 1.67 g=L] JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 / 979 J Environ Eng 2013.139:975-985 30 [paraquat] (ppm) 25 Sample-9 Sample-10 Sample-11 Sample-12 "Sample-13 (dried)" "Sample-14 (dried)" "Sample-15( dried)" "Sample-16 (dried)" Activated Carbon 20 15 10 0 10 15 Time (sec) 20 25 30 Fig Measured paraquat concentrations after adding silica Samples 9–16 [samples were collected up to 30 s after adding silica particles; sample aliquots were collected at 5-s intervals; solution pH was 8.04 in borate buffer; silica concentration in 25-ppm paraquat solution (initial concentration) was 1.67 g=L] observation time frame of 48 h (Fig 3) Paraquat adsorption after 48 h was found to be the greatest onto activated-carbon powders; however, more than 96% of paraquat was removed from solution with unmodified silica Samples 3, 4, and 6, and modified silica Samples 5, 8, 13, and 16 (Table 1) The BET-N2 measurements enabled comparison of the adsorption data of some selected samples (Samples 1–6, Sample 10, and activated carbon) for normalized surface area (i.e., paraquat adsorbed per unit surface area) (Table 1) For a unit area of adsorption, the short-term performance of the samples can be ranked as Sample > Sample > Sample > Sample > Sample > Sample > Sample 10 > activated carbon, and the long-term interaction can be ranked as Sample > Sample 10 > Sample > Sample > Sample > Sample > Sample > activated carbon: Paraquat adsorbed per unit area was maximum for Sample (42.5 g=m2 after 30 s and 91.2 μg=m2 after 48 h) and the lowest Sample-1 Sample-4 Sample-7 Sample-10 "Sample-13 (dried)" "Sample-16 (dried)" 30 25 [paraquat] (ppm) Downloaded from ascelibrary.org by New York University on 05/17/15 Copyright ASCE For personal use only; all rights reserved Sample-2 Sample-5 Sample-8 Sample-11 "Sample-14 (dried)" Activated Carbon Sample-3 Sample-6 Sample-9 Sample-12 "Sample-15( dried)" 20 15 10 10 100 1,000 Time (sec) 10,000 100,000 1,000,000 Fig Adsorption kinetics of paraquat onto tested particles over 48 h, with time plotted on log scale [pH of solution was 8.06 (using borate buffer); activated carbon or silica in 25-ppm paraquat solution (initial concentration) was 1.67 g=L] for activated carbon (11.6 μg=m2 after 30 s and 16.9 μg=m2 after 48 h) Although activated carbon asymptotically adsorbed all paraquat present, it is not obvious that the long-term behavior is sufficiently better than silica to warrant its use, particularly given its poorer short-term performance An apparently exponential trend in the adsorption time series was evident for Samples 10, 11, and 13 in which removal rate for Sample 13 was relatively rapid (Fig 2) Initially, paraquatadsorption rate by Sample 10 was the slowest; however, total adsorptive paraquat removal by the sample was found to be higher than that by Samples 11, 12, and 15 (Fig and Table 1) As informed by the supplier, Sample 10 is granulated and has low surface area by configuration, and therefore the sample offers fewer available adsorption sites but more particles in solution compared with the same quantity of other silica particles with larger surface areas (Table 1) Considering the maximum paraquat-adsorption and paraquatremoval rates, Sample (which is an unmodified silica) exhibited the best performance among all the tested silica in that 93% of paraquat was removed from the heterogeneous solution within the first 150 s after adding the silica Investigation of adsorption behavior at different initial paraquat concentrations indicates that the maximum amount of paraquat that can be adsorbed by a silica sample is dependent on the paraquat concentration of the solutions Fig 4(a) presents a comparison of the measured decrease in paraquat after adding silica (Samples 1–8) With increasing initial concentration, the percentage of paraquat removal was found to decrease, whereas the total amount of contaminant adsorbed by a fixed dose of silica (1.67 g=L) increased Further investigation of the adsorption behavior using Sample at different initial paraquat concentrations reveals that the paraquat uptake by silica, at least within the observed time frame (1 min), is dominated by equilibrium phenomena until the surface of the adsorbent is saturated [Fig 4(b)] Approximately 90% paraquat-removal efficiency was achieved in for paraquat solutions of different initial concentrations up to 27 ppm, but beyond initial concentrations of 27 ppm, saturation of the silica surface started to control the paraquat-uptake process Because loading of the silica particles was kept constant for all concentrations, increasing the initial solution concentration at this stage actually increased the number of paraquat molecules competing for a single adsorption site Consequently, there is a reduction in instant adsorption (e.g., percentage of paraquat adsorbed); however, the mass transfer rate of the solute is enhanced because of a higherconcentration gradient between the bulk solution and adsorption interface resulting in an increase of total paraquat adsorption onto the adsorbents Fig 5(a) exhibits the chromatograms for the control sample (100-ppm paraquat solution) For all the initially received silica samples (Samples 1–8) except Sample [treated with polyethylene glycol (PEG)], the chromatograms were similar to Fig 5(a) for all the silica concentrations (1.3, 1.67, and g=L) attempted and indicate that the samples not react with paraquat The chromatogram for Sample [Fig 5(b)] displayed additional elution peaks from retention time t ¼ 2.54–4.08 The combined mass spectrum, taken from t ¼ 3.61–4.08 min, displays mass peaks at m=z values higher than that of the precursor ion, ẵPQỵ , and thus indicates the inclusion of additional compounds into the solution, most likely from the treated silica surface (Sample 5) The presence of the mass peaks at regular m=z intervals of 44.0 further confirms that oligomers of PEG are present, because the monomer of PEG (-CH2 -O-CH2 ) has a fragment mass of 44.0 Dalton 980 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 J Environ Eng 2013.139:975-985 35 Sample Sample Sample Sample Sample Sample Sample Sample demonstrated in Fig 6(a) The plot demonstrates a poor fit of the Langmuir isotherm model to the experimental data [Fig 6(a)] Because the nature of solute adsorption onto particulates is more complicated in a solution system compared with gas adsorption on a homogeneous solid surface, factors like solute–solvent interaction, competitive adsorption between a solute and the solvent, surface heterogeneity, and solution conditions can come into play (Zuyi and Taiwei 2000) The Langmuir gas isotherm model failed to describe the paraquat-uptake process onto silica Fitting with the competitive binding model was then attempted Paraquat-adsorption kinetics were modeled, assuming that all the surface adsorption sites are occupied with water before paraquat adsorption occurs The model was found to be in good agreement with the experimental data for Sample 10 through the first 1,200 s [Fig 6(b)] assuming 72% surface coverage (θ) at equilibrium (= 0.72) and constant K ¼ 0.03 s−1 The consistency of the modeling and experimental results suggests that the adsorption kinetics of the silica particles are governed by competitive adsorption of water and paraquat However, comparison of the competitive binding model prediction with long-time experimental data over 48 h exhibits that the model deviates from experimental data over time 30 25 20 15 10 15 (a) 25 35 Initial Concentration (ppm) 45 100 90 80 % Paraquat Removal 70 60 50 % Paraquat removal 90% removal 40 25 Experimental Competitive Binding Model 30 Langmuir Model 20 10 (b) 10 20 30 40 Initial Concentration of Paraquat 50 60 Fig Effect of initial concentration on paraquat adsorption: (a) comparison among Samples 1–8; (b) detailed analysis for Sample [paraquat] (ppm) 20 15 10 20 40 60 (a) 80 Time (sec) 100 [paraquat] (ppm) Downloaded from ascelibrary.org by New York University on 05/17/15 Copyright ASCE For personal use only; all rights reserved Decrease in Paraquat Concentration (ppm) 40 100 120 140 Experimental Competitve Binding Model Langmuir Model 10 1 (b) Fig Chromatograms: (a) aqueous solution of paraquat dichloride; (b) paraquat solution mixed with Sample (initial concentration of paraquat dichloride solution was 100 ppm) Model Predictions A comparison between mathematical model predictions and experimental adsorption data for paraquat uptake onto Sample 10 is 10 100 1,000 Time (sec) 10,000 100,000 1,000,000 Fig Comparison between mathematical model predictions and experimentally obtained adsorption data for Sample 10, with time plotted in logarithmic scale: (a) over 150 s; (b) over 48 h [for all experiments, silica in 25-ppm paraquat solution (initial concentration) was 1.67 g=L; exponential Langmuir model fit decays more slowly than experimental data; competitive model fits well with experimental data collected for first h, but actual paraquat concentration is much lower than model predicts after 48 h] JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 / 981 J Environ Eng 2013.139:975-985 kfast kslow A ẳ ỵ t þ kfast t þ kslow t where tc = critical time after which second process begins; and B was considered to be constant Fig demonstrates the fitting curve, obtained after incorporating the HSDM into the adsorption model When only one competitive process was considered, the fitting was similar to the competitive binding curve shown in Fig 6(b); however, the total surface coverage (θ ¼ 0.33) was found to be lower in this case Because the inclusion of the diffusion equation enabled the model to consider additional adsorption inside the pores, the total adsorption predicted can be the same, even in the presence of the fewer available surface sites [Fig 8(a)] Among the models applied, the two-stage adsorption model was found to be the only method for which the fitting predictions were in the vicinity of the experimental adsorption data [Fig 8(b)] at long times Addition of H2 O2 to Paraquat Solutions Because adsorption was found to be the only active sequestration or degradation mechanism during paraquat–silica interactions, the addition of exogenous oxidant (H2 O2 ) to heterogeneous 100 Experimental One competitive binding model in combination with HSDM [paraquat] (ppm) 10 ð9Þ where kfirst and kslow = rate constants for first and second competitive binding process; and kslow ¼ Bðt − tc Þ ð10Þ 1 (a) 10 100 100 1,000 Time (sec) 10,000 100,000 1,000,000 Experimental Two-step adsorption model in combination with HSDM [paraquat] (ppm) 4.50 Solid Phase Concentration q(mg/g) Downloaded from ascelibrary.org by New York University on 05/17/15 Copyright ASCE For personal use only; all rights reserved The final paraquat concentration was found to be lower than the extrapolated model fit after 48 h [Fig 6(b)] The initial understanding was that model predictions deviated from long-time (i.e., after h and up to 48 h) adsorption data because the competitive binding model does not consider the presence of pores on the silica particles Therefore, the molecules adsorbed onto the surface of the silica particles may diffuse into the adsorbents over time, and thus the particles actually offer available surface sites for more adsorption than the model estimates at later stages of interaction However, when the homogeneous solid diffusion equation was applied in combination with the competitive binding model, the concentration variation of paraquat inside the pore indicates that the paraquat transport to the pore is largely complete within the first 40 s (i.e., for Sample 1) of paraquat–silica interaction (Fig 7) Fig demonstrates the variation of concentration gradient of paraquat inside the pore for Sample at different time steps The results elucidate that paraquat diffusion through the pores is not controlling the long-time adsorption behavior, and the increased paraquat adsorption is likely attributable to the availability of more surface sites at long times Therefore, to model the adsorption data, paraquat adsorption onto the silica was then modeled by assuming a two-stage adsorption process—namely, a fast competitive binding phase in combination with HSDM—to consider external and internal diffusion, and a slow phase controlled by intraparticle diffusion The idea proposed in this paper is similar to the double exponential model proposed in the literature to model long-time adsorption of copper and lead onto activated carbon (Wilczak and Keinath 1993) and to the observation made on diffusion of protein in membranes (Jacobson et al 1987) The competitive binding process equation considered in this case is as follows: 4.25 4.00 3.75 t=10s t=20s t=40s t=150s 10 1 3.50 (b) 3.25 12 15 r (µm) Fig Solid-phase concentration variation along radius of Sample at different time (t) steps (r ¼ and 16 μm denotes center and surface of silica particles, respectively) 10 100 1,000 Time (sec) 10,000 100,000 1,000,000 Fig Comparison between: (a) MATLAB computation results for one competitive binding process in combination with HSDM and experimental data [fitting parameters are k1 ẳ 0.32s1 ị), ẳ 0.33, and Ds ¼ × 10−5 m2 =s]; (b) two-step adsorption model in combination with HSDM and experimental adsorption data [fitting paramters are kfast ẳ 0.32s1 ị), tc ẳ 150 s, B ẳ 1.3 ì 1010 s2 ị, ẳ 0.33, and Ds ẳ ì 105 m2 =s 982 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 J Environ Eng 2013.139:975-985 2.0 0.48 mM paraquat with 2.4 mM hydrogen peroxide 0.48 mM paraquat with 14.2 mM hydrogen peroxide 0.48 mM paraquat with 25.9 mM hydrogen peroxide 0.48 mM paraquat with 37.5 mM hydrogen peroxide 0.48 mM paraquat with 48.9 mM hydrogen peroxide 0.48 mM paraquat with 60.1 mM hydrogen peroxide 0.48 mM paraquat with 71.2 mM hydrogen peroxide 0.48 mM paraquat with 82.1 mM hydrogen peroxide 0.48 mM paraquat with 93.0 mM hydrogen peroxide 98.3 mM hydrogen peroxide 1.5 Absorbance peak corresponding to paraquat became less evident with increasing H2 O2 concentrations in solutions (Fig 10) The apparent shift observed was actually attributable to the relative position of the paraquat- and H2 O2 -absorption peaks, such that increasing peroxide absorption raised the short-wavelength band wing absorption and shifted the wavelength of peak absorption In addition, H2 O2 modifies the solution hydrogen bonding matrix, which ultimately shifts the absorption band of water to longer wavelengths (Higashi et al 2008) The results point to the conclusion that H2 O2 alone does not have any significant interaction with dicationic paraquat The LC-MS analysis of H2 O2 -added paraquat solutions with or without the silica (Fig 11) was also in agreement with the spectrometric results No decomposition was evident for any aliquots tested To identify whether the absence of any reaction was attributable to the reduction of surface active sites (before adding H2 O2 ) on silica by preadsorbed paraquat, a silica sample (Sample 2) and H2 O2 were added simultaneously to paraquat solution However, no change in the chromatogram of Fig 11 was observed, which indicates that the heterogeneous solution remains nonreactive, even in the presence of putatively reactive surface sites 1.0 0.5 0.0 210 230 250 270 Wavelength (nm) 290 310 Fig Absorbance spectrum of H2 O2 -added paraquat solutions for varying H2 O2 concentrations; ẵparaquat ẳ 0.48 mM; labels indicate concentration of H2 O2 added (solution pH was 5.87 in acetate buffer) Fig 11 Chromatogram of 100-ppm paraquat solutions when mixed with approximately 50 mM H2 O2 and no silica; similar chromatogram were observed by adding both H2 O2 and silica to paraquat solutions 1.0 0.48 mM paraquat with 2.4 mM hydrogen peroxide 0.48 mM paraquat with 8.3 mM hydrogen peroxide 0.48 mM paraquat with 14.2 mM hydrogen peroxide 0.48 mM paraquat with 20.1 mM hydrogen peroxide 0.48 mM paraquat with 25.9 mM hydrogen peroxide 0.48 mM paraquat with 31.7 mM hydrogen peroxide 0.48 mM paraquat with 37.5 mM hydrogen peroxide 0.48 mM paraquat with 43.2 mM hydrogen peroxide 0.48 mM paraquat with 48.9 mM hydrogen peroxide 0.48 mM paraquat with 54.5 mM hydrogen peroxide 0.48 mM paraquat with 60.1 mM hydrogen peroxide 0.48 mM paraquat with 65.7 mM hydrogen peroxide 0.48 mM paraquat with 71.2 mM hydrogen peroxide 0.48 mM paraquat with 76.7 mM hydrogen peroxide 0.48 mM paraquat with 82.1 mM hydrogen peroxide 0.48 mM paraquat with 87.6 mM hydrogen peroxide 0.48 mM paraquat with 93.0 mM hydrogen peroxide 0.48 mM paraquat with 98.3 mM hydrogen peroxide 0.9 0.8 Corrected Absorbance Downloaded from ascelibrary.org by New York University on 05/17/15 Copyright ASCE For personal use only; all rights reserved proxy solutions (with silica) was attempted to provide a reactant so that silica might be able to catalytically decompose paraquat Initially, the interaction between the compounds (i.e., H2 O2 and paraquat) was investigated spectrometrically by introducing varying concentrations of H2 O2 to 0.48 mM of buffered paraquat solutions (Fig 9) The idea was to look for unstable complex formation and better understand the nature and stoichiometry of interaction of paraquat and H2 O2 so that viable techniques to catalyze paraquat decomposition could later be attempted As illustrated in Fig 9, the absorbance peak of the solution shifted to lower wavelengths for higher concentrations of H2 O2 in solution, which was initially interpreted as signifying the presence of light absorbing unknown compounds However, an alternative explanation for the peak shift in Fig was that the shift of the absorption peak was attributable to addition of H2 O2 and an apparent shift independent of any association or reaction Therefore, the absorption spectra in Fig were replotted, subtracting the putative absorbance of H2 O2 (Fig 9) No additional absorbance peak was identified, and the shift of the absorption 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 200 250 300 Wavelength (nm) 350 400 Fig 10 Corrected absorbance spectrum (i.e., corrected absorbance = absorbance of H2 O2 -added paraquat solution minus computed, scaled absorbance of H2 O2 in that solution) paraquat solutions for varying H2 O2 concentrations; ẵparaquat ẳ 0.48 mM; labels indicate concentration of H2 O2 added (solution pH was 5.87 in acetate buffer); only shift is attributable to offset in baseline, which may be attributable to relative position of paraquat and H2 O2 absorption peaks or modification of solution hydrogen bonding matrix by H2 O2 JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 / 983 J Environ Eng 2013.139:975-985 Downloaded from ascelibrary.org by New York University on 05/17/15 Copyright ASCE For personal use only; all rights reserved Conclusion Silica was found to be a promising adsorbent for watertreatment applications in which maximum removal of paraquat within a limited time of interaction is critical Although activated carbon sequestered paraquat more effectively than silica at long times (i.e., after h and up to 48 h), considering both constant mass and constant surface area approach in loading the adsorbents, the best performance in short-term (i.e., at least up to 150 s) interaction was achieved by using unmodified silicas When the constant mass approach was adopted for loading the adsorbents, all the unmodified silica samples except Sample were found to be good adsorbents for time periods relevant to water purification, and percentage of paraquat removal within 30 s was higher than that achieved by activated-carbon sample for three (Samples 3, 4, and 6) of the five unmodified silicas In fact, the unmodified silica Samples and outclassed all the tested samples when short-term adsorption data were compared After long-term interaction, at least 90% paraquat removal was achieved for all the unmodified silicas The paraquat adsorption observed onto the surfaces of different base materials (e.g., silica) can certainly aid removal of the contaminant when appropriate reagents are incorporated into the adsorbent, thus holding the promise to optimize efficiency of a regenerable membrane-based watertreatment system with a low carbon footprint Silica was also found to be a suitable adsorbent over activated carbon for both short-time and long-time application when size or area of the membrane system is an important design consideration Normalized adsorption data exhibit that, for the same surface area of the adsorbents, all the silicas analyzed (Samples 1–6 and 10) adsorb more paraquat than the activated carbon during the entire observation period (i.e., 48 h) Concentration-dependency data demonstrate that paraquat uptake is equilibrium limited for the first after adding fresh silica The results suggest that when immediate protection against paraquat contamination is the primary concern, effective cleanup of the paraquat solution is possible by simply passing the solution through a series of membranes coated with clean unmodified silica Agreement between short-time (i.e., adsorption data up to 1,200 s) experimental adsorption data and single-step competitive binding model predictions also indicates competitive adsorption of both paraquat and water molecules onto silica particles Among the adsorption models attempted, predictions using a two-step competitive binding approach with consideration of the homogeneous solid diffusion model were found to be in the vicinity of the longtime adsorption data However, neither the modified nor unmodified silica reacted with dicationic paraquat, either in the presence or the absence of H2 O2 in solution Future work on this research will be to utilize the adsorption results obtained for the materials in suspension to investigate paraquat removal using membranes containing silica so that the best paraquat-adsorbing silica candidate can be identified, and mechanisms of decomposing or sequestering paraquat can be successfully implemented Acknowledgments This material is based upon work supported by the Engineer Research and Development Center–Construction Engineering Regulation Laboratory (ERDC-CERL) under Contract No W9132T-09-C-0046 Work was further supported and, in part, collaboratively guided by PPG Industries Sincere acknowledgment 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of particle size of activated clay on the adsorption of paraquat from aqueous solution.” J Colloid Interface Sci., 263(1), 29–34 Veliev, E V., Öztürk, T., Veli, S., and Fatullayev, A G (2006) “Application of diffusion model for adsorption of azo reactive dye on pumice.” Pol J Environ Stud., 15(2), 347–353 Wilczak, A., and Keinath, T M (1993) “Kinetics of sorption and desorption of copper(II) and lead (II) on activated carbon.” Water Environ Res., 65(3), 238–244 Yuen, S H., Bagness, J E., and Myles, D (1967) “Spectrophotometric determination of diquat and paraquat in aqueous herbicide formulations.” Analyst, 92(1095), 375–381 Zuyi, T., and Taiwei, C (2000) “On the applicability of the Langmuir equation to estimation of adsorption equilibrium constants on a powdered solid from aqueous solution.” J Colloid Interface Sci., 231(1), 8–12 JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 / 985 J Environ Eng 2013.139:975-985 ... information such as BET-N2 —specific surface area measurements, volume of 25-ppm paraquat used (30 mL), and loading of adsorbents (1.67 g=L) Silica Silica Silica Silica Silica Silica Silica Silica... maximum paraquat- adsorption and paraquatremoval rates, Sample (which is an unmodified silica) exhibited the best performance among all the tested silica in that 93% of paraquat was removed from. .. spectrophotometer for UV absorption of paraquat at 258 nm was performed at pH ¼ 8.06 (a 0.025-M borate buffer) A calibration curve for H2 O2 at the peak wavelength of paraquat absorption (258