ARTICLE pubs.acs.org/Langmuir
Characterization of Adsorption of Humic Acid onto Alumina using Quartz Crystal Microbalance with Dissipation Mingquan Yan,*,† Chunxia Liu,‡ Dongsheng Wang,§ Jinren Ni,† and Jixia Cheng‡ †
Department of Environmental Engineering, Peking University, The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China ‡ College of Environmental Sciences and Engineering, Chang’an University, Xi’an, Shanxi, 710064, China § State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, CAS, POB 2871, Beijing 100085, China
bS Supporting Information ABSTRACT: In this paper, a quartz crystal microbalance with dissipation monitoring (QCM-D) is used to investigate humic acid (HA) adsorption onto alumina (Al2O3). The amount of adsorption and layer structures of HA were determined by the real-time monitoring of resonance frequency and energy dissipation changes (Δf and ΔD). The effect of HA concentration, HA molecular characteristics (molecular weight and polarity), and pH on HA adsorption onto Al2O3 were investigated. The mass of HA adsorption increases as the concentration of HA increases. The masses are about 24, 60, and 87 ng cm-2 as the concentration of DOC is 1.0, 4.85, and 92.0 mg L-1, respectively. The adsorbed layer of HA is more nonrigid, and the mass of HA adsorption is higher at weakly acidic pH values. It was 20, 80, 65, and 45 ng cm-2 at pH values of 4.5, 5.5, 6.5, and 8.0, respectively. This reveals that efficient HA removal by coagulation at weakly acidic pH values is not just due to the hydrolysis of Al ions as previously presumed. The adsorbed layer of hydrophobic HA is more nonrigid than hydrophobic HA (fractionated by Amberlite XAD-8 resin), and the mass adsorption for the hydrophobic fraction is about four times higher than the hydrophilic fraction (120 ng cm-2 and 30 ng cm-2). The method is of value in the research to establish a quantified calculation model for the coagulation process.
’ INTRODUCTION Natural organic matter (NOM), which is produced mainly from the decomposition of plant and animal biomass,1,2 is a mixture of chemically complex polyelectrolytes of varying molecular weight and functional groups. More attention has been paid to NOM removal because it is the precursor for disinfection byproduct (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs), which are known carcinogens.3,4 Coagulation is the most popular and cost-effective choice to eliminate these DBP-precursors,5-7 and Al-based coagulants are widely used in water plants as the coagulant. NOM is thought to be removed during coagulation by two general mechanisms: formation of insoluble metal organic complexes or adsorption onto a metal hydroxide floc.4,8-10 The latter is especially important for dissolved organic matter. Regardless of the mechanism, the interaction of an Al oxy-hydroxide surface and NOM is the key factor to determine coagulation performance. Researchers have employed many methods to illustrate the mechanism of the adsorption of NOM onto the hydrolyzed product of alum and alumina: batch experiments, Al and fulvic acid (FA) titrations, acid-base titrations, IR techniques, etc. Santhiya, et al.,11 Jucker, et al.,12 and Browne et al.13 used batch or r 2011 American Chemical Society
column isotherm experiments to determine the amounts of NOM adsorbed by comparing solution concentrations before and after adsorption. On the basis of zeta-potential and acidbase titration measurements, researchers hypothesized that electrostatic interactions and a ligand exchange process occur simultaneously when humics adsorb to a preformed floc. A correlation between removal efficiency and the number and ionization of NOM functional groups was found, and the removal of material generally increased with a greater number of functional groups and thus greater molecular weight.14-18 IR spectroscopy has also been used to qualitatively identify key functional groups and to obtain the structures of polymers interacting with mineral surfaces.12,19 Because comparison of batch experiments can involve significant uncertainties because humic substances have low affinities to surfaces, quartz crystal microbalance with dissipation monitoring (QCM-D) is a useful technique in these low-affinity cases. QCM-D can quantify relatively low levels of adsorption Received: October 21, 2010 Revised: January 1, 2011 Published: July 21, 2011 9860
dx.doi.org/10.1021/la1042102 | Langmuir 2011, 27, 9860–9865
Langmuir
ARTICLE
(ng cm-2) with real-time monitoring.20 Recently, this QCM-D technique was used in environmental studies to detect the adsorption of specific organic compounds such as dispersants21,22 and bacterial extracellular polysaccharides20,23 due to the method’s high sensitivity. The goal of this study was to investigate the adsorption of HA onto alumina using QCM-D. Typical HA extracted from Yellow River sediment was used, and resin adsorption and ultrafiltration were used also to further investigate the effects of HA polarity and molecular weight characteristics on its adsorption onto alumina. The results could prove useful in subsequent research to establish a quantified calculation model on the coagulation process.
’ MATERIALS AND METHODS QCM-D Experiments. QCM-D technology has been described elsewhere.20,21,24 The general QCM principle is to apply an ac voltage in the MHz range across an AT-cut piezoelectric quartz crystal to record the resonance frequency of the crystal. In QCM-D, the ac voltage is pulsed to monitor the damping or “ring-down” of crystal oscillation. The QCM-D response is sensitive to any mass change of the quartz crystal, including the mass of bound water that is associated with the adsorbed solute. The density and thickness of the adsorbed layer will have an effect on the response, as well as on the viscoelastic properties of adsorbed species. For flat, uniform, and rigid films, the change in resonance frequency, Δf = f - f0, is directly proportional to the adsorbed mass (Δm), which is normally referred to as the Sauerbrey equation. Δm ¼ -
CQCM Δf n
ð1Þ
where CQCM is the mass sensitivity constant (17.7 ng cm-2 Hz at f = 5 MHz), and n is the overtone number (1, 3, 5, 7, 9, ...; the number of resonance for output frequency of a quartz oscillator). On the basis of this relationship, a decreasing resonance frequency corresponds to a proportional mass uptake on the sensor crystal surface. The conditions for this relationship to be valid are as follows: (i) the adsorbed mass is evenly distributed on the crystal, (ii) the mass is much smaller than the crystal mass itself, and (iii) the mass is rigid enough not to deform during crystal oscillation. Thus, for soft and viscoelastic materials, the linear relationship between frequency and mass is not necessarily valid because the materials do not fully couple to the crystal oscillations. From the decay of the crystal oscillations in a QCM-D experiment, the energy dissipation (D, dissipation factor) is simultaneously monitored to obtain the viscoelastic properties of the material using the following equation: D¼
Edissipated 2πEstored
ð2Þ
where Ddissipated is the energy lost during one oscillation, and Estored is the energy stored in the oscillating circuit. For an adsorbed layer with high rigidity, no change in dissipation will be observed as a function of adsorption. However, for an adsorbed viscoelastic layer, the energy dissipating through the layer will increase. Therefore, by observing the change in dissipation, ΔD = D - D0, a semiquantitative measure of the relative stiffness or conformation of an adsorbed layer may be determined. Because of the limitations of the Sauerbrey equation (eq 1), the Voigtbased viscoelastic model has been used to obtain more accurate adsorbed mass values.25,26 In the Voigt-based model, the frequency change (Δf) and energy dissipation (ΔD) of crystal oscillations in a QCM-D experiment are related to the physical parameters of an adsorbed film (e.g., thickness, viscosity, shear modulus, and density). Hence, the viscoelastic parameters and mass of the adsorbed film can be
determined by fitting with the simultaneously measured values of Δf and ΔD at multiple overtones. The adsorbed layer can be represented by a homogeneous viscoelastic film, which is characterized by a shear viscosity, ηf, and a shear modulus, μf, as well as a thickness and a density. The ΔD/Δf ratio can give information on adsorbed layer. For example, a high ΔD/Δf ratio corresponds to a relatively nonrigid open structure, whereas a low ratio corresponds to a stiffer and more compact structure where the adsorbed mass induces relatively low energy dissipation. Additionally, a change in the slope of ΔD/Δf indicates coverage-induced structural changes in the adsorbed layer. A decrease in the slope is an indication that the layer has become more rigid due to increased packing density.25,26 A QCM-D E1 system (Q-Sense AB, Gothenburg, Sweden) was utilized to examine the adsorption of HA onto Al2O3 surfaces. QCM-D experiments were performed with 5 MHz AT-cut quartz sensor crystals with Al2O3-coated surfaces (batch 081128-1). Prior to measuring, the crystal was cleaned by submerging it into an aqueous solution of a nonionic surfactant (Sabopal LM7). It was then rinsed in deionized water followed by ethanol and put in an ultrasonic bath for 10 min. The crystal was rinsed again in deionized water, dried over flowing nitrogen gas, and then exposed to UV/ozone for 10 min. The whole procedure was repeated once again, and the crystals were subsequently put in sterile containers for future use. The temperature of the measurement chamber was kept at 25 ( 0.1 °C. In a typical experiment, a baseline was first established for deionized water. The changes in frequency and dissipation upon adsorption at various overtones were monitored simultaneously throughout the experiment. A commercial software program (Q-Tools, Q-Sense AB, G€oteborg, Sweden) developed for QCM-D was used for the calculations. In the Voigt modeling, bulk density and bulk viscosity values of 1.0 g cm-3 and 0.001 Pa s, respectively, were used for all fits. The layer density was approximated to 1.0 g cm-3. HA Characteristics. A well-characterized commercial source of HA (extracted from Yellow River sediment) was used in the experiment.27 It was dissolved into deionized water and then filtered using a 0.45 μm pore size filter membrane to remove particulate HA. The HA is further fractionated by ultrafiltration and resin adsorption. Hollow-fiber modules and ultrafiltration (UF) membranes (A/G Technology) with nominal molecular weight cut-offs of 30, 3, and 1 kDa were used to sequentially fractionate dissolved organic matter (DOM). The membranes were made of cellulose derivatives with a total surface area of 24 cm2. The applied pressure through the membranes ranged from 250 to 350 kPa. The total organic carbon (TOC) of the effluent from each membrane and of the raw water were measured to determine the content of each cumulative fraction. The percentages of HA with molecular weight of >30 kDa, 3-30 kDa, 1-3 kDa, and
