Comparison of natural organic matter adsorption capacities of super-powdered activated carbon and powdered activated Carbon

We examined the natural organic matter (NOM) adsorption characteristics of super-powdered activated carbon (S-PAC) produced by pulverizing commercially available, normal PAC to a submicron particle size range. The adsorption capacities of S-PAC for NOM and polystyrene sulfonates (PSS) with molecular weights (MWs) of 1.1, 1.8, and 4.6 kDa, which we used as model compounds, were considerably higher than those of PAC. The adsorption capacity increases were observed for all five types of carbon tested (two wood-based, two coconut-based, and one coal-based carbon). The adsorption capacities of S-PAC and PAC for polyethylene glycols (PEGs) with MWs of 0.3 and 1.0 were the same. The adsorption capacities of S-PAC for PEGs with MWs of 3.0 and 8.0 kDa were slightly higher than the adsorption capacities of PAC, but the difference in adsorption capacity was not as large as that observed for NOM and the PSSs, even though the MW ranges of the adsorbates were similar. We concluded that the adsorption capacity differences between S-PAC and PAC observed for NOM and PSSs were due to the difference in particle size between the two carbons, rather than to differences in internal pore size or structure, to differences in activation, or to non-attainment of equilibrium that resulted from the change in particle size. The difference in adsorption capacity between S-PAC and PAC was larger for NOM with a high specific UV absorbance (SUVA) value than for low-SUVA NOM. The larger adsorption capacities of S-PAC compared with PAC were explained by the larger specific external surface area per unit mass. We hypothesize that a larger fraction of the internal pore volume is accessible with carbon of smaller particle size because the NOM and PSS molecules preferentially adsorb near the outer surface of the particle and therefore do not completely penetrate the adsorbent particle.     

Enhanced mitigation of para-chlorophenol using stratified activated carbon adsorption columns

The adsorptive removal of toxic para-chlorophenol using activated carbon adsorption columns is a proven effective engineering process. This paper examined the possibility to stratify an adsorbent bed into layers, in order to enhance the adsorption process performance in terms of increased column service time and adsorbent bed saturation. Four different types of fixed-bed adsorption columns are used and compared under the same operating conditions, but with the variation of column geometry and activated carbon particle size stratification. The Type 3 column - a cylindrical column with particle stratification packing, is found to be the most efficient choice, as the extent of column service time and adsorbent bed saturation are the largest. This could eventually decrease the frequency of adsorbent replacement/regeneration and hence reduce the operating cost of the fixed-bed adsorption process. The Homogeneous Surface Diffusion Model (HSDM) was applied successfully to describe the dynamic adsorption of para-chlorophenol onto Filtrasorb 400 (F400) activated carbon in different types of columns. The Redlich-Peterson isotherm model equation, an experimentally derived external mass transfer correlation and a constant surface diffusivity are used in the HSDM. The optimised surface diffusivity of para-chlorophenol is found to be 1.20E-8 cm²/s, which is in good agreement with other phenolics/F400 carbon diffusing systems in literature.     

Effects of super-powdered activated carbon pretreatment on coagulation and trans-membrane pressure buildup during microfiltration

As a pretreatment for membrane microfiltration (MF), the use of powdered activated carbon (PAC) with a particle size much smaller than that of conventional PAC (super-powdered PAC, or S-PAC) has been proposed to enhance the removal of dissolved substances. In this paper, another advantage of S-PAC as a pretreatment for MF is described: the use of S-PAC attenuates trans-membrane pressure increases during the filtration operation. The floc particles that formed during coagulation preceded by S-PAC pretreatment were larger and more porous than the floc particles formed during coagulation preceded by PAC pretreatment and those formed during coagulation without pretreatment. This result was due to increased particle–particle collision frequency and better removal of natural organic matter, which inhibits coagulation by consuming coagulant, before the coagulation reaction. The caked fouling layer that built up on the membrane surface was thus more permeable with S-PAC than with normal PAC. Both physically reversible and irreversible membrane foulings were reduced, and more stable filtration was accomplished with S-PAC pretreatment.     

Modeling high adsorption capacity and kinetics of organic macromolecules on super-powdered activated carbon

The capacity to adsorb natural organic matter (NOM) and polystyrene sulfonates (PSSs) on small particle-size activated carbon (super-powdered activated carbon, SPAC) is higher than that on larger particle-size activated carbon (powdered-activated carbon, PAC). Increased adsorption capacity is likely attributable to the larger external surface area because the NOM and PSS molecules do not completely penetrate the adsorbent particle; they preferentially adsorb near the outer surface of the particle. In this study, we propose a new isotherm equation, the Shell Adsorption Model (SAM), to explain the higher adsorption capacity on smaller adsorbent particles and to describe quantitatively adsorption isotherms of activated carbons of different particle sizes: PAC and SPAC. The SAM was verified with the experimental data of PSS adsorption kinetics as well as equilibrium. SAM successfully characterized PSS adsorption isotherm data for SPACs and PAC simultaneously with the same model parameters. When SAM was incorporated into an adsorption kinetic model, kinetic decay curves for PSSs adsorbing onto activated carbons of different particle sizes could be simultaneously described with a single kinetics parameter value. On the other hand, when SAM was not incorporated into such an adsorption kinetic model and instead isotherms were described by the Freundlich model, the kinetic decay curves were not well described. The success of the SAM further supports the adsorption mechanism of PSSs preferentially adsorbing near the outer surface of activated carbon particles.     

Intraparticle diffusion and adsorption of arsenate onto granular ferric hydroxide (GFH)

Porous iron oxides are being evaluated and selected for arsenic removal in potable water systems. Granular ferric hydroxide, a typical porous iron adsorbent, is commercially available and frequently considered in evaluation of arsenic removal methods. GFH is a highly porous (micropore volume approximately 0.0394+/-0.0056 cm(3)g(-1), mesopore volume approximately 0.0995+/-0.0096 cm(3)g(-1)) adsorbent with a BET surface area of 235+/-8 m(2)g(-1). The purpose of this paper is to quantify arsenate adsorption kinetics on GFH and to determine if intraparticle diffusion is a rate-limiting step for arsenic removal in packed-bed treatment systems. Data from bottle-point isotherm and differential column batch reactor (DCBR) experiments were used to estimate Freundlich isotherm parameters (K and 1/n) as well as kinetic parameters describing mass transfer resistances due to film diffusion (k(f)) and intraparticle surface diffusion (D(s)). The pseudo-equilibrium (18 days of contact time) arsenate adsorption density at pH 7 was 8 microg As/mg dry GFH at a liquid phase arsenate concentration of 10 microg As/L. The homogeneous surface diffusion model (HSDM) was used to describe the DCBR data. A non-linear relationship (D(S)=3.0(-9) x R(p)(1.4)) was observed between D(s) and GFH particle radius (R(P)) with D(s) values ranging from 2.98 x 10(-12) cm(2)s(-1) for the smallest GFH mesh size (100 x 140) to 64 x 10(-11) cm(2)s(-1) for the largest GFH mesh size (10 x 30). The rate-limiting process of intraparticle surface diffusion for arsenate adsorption by porous iron oxides appears analogous to organic compound adsorption by activated carbon despite differences in adsorption mechanisms (inner-sphere complexes for As versus hydrophobic interactions for organic contaminants). The findings are discussed in the context of intraparticle surface diffusion affecting packed-bed treatment system design and application of rapid small-scale column tests (RSSCTs) to simulate the performance of pilot- or full-scale systems at the bench-scale.     

Pore blockage effect of NOM on atrazine adsorption kinetics of PAC: the roles of PAC pore size distribution and NOM molecular weight

Natural organic matter (NOM) in natural water has been found to have negative effects on the adsorption of various trace organic compounds by activated carbon through two major mechanisms: direct competition for sites and pore blockage. In this study, the pore blockage effect of NOM on atrazine adsorption kinetics was investigated. Two types of powdered activated carbon (PAC) and three natural waters were tested to determine the roles of PAC pore size distribution and NOM molecular weight distribution in the pore blockage mechanism. When PAC was preloaded with natural water, the pore blockage effect of the NOM was found to cause a reduction of up to more than two orders of magnitude in the surface diffusion rate of atrazine compared to simultaneous adsorption of atrazine and NOM with fresh PAC. The surface diffusion coefficient of atrazine for preloaded PAC decreased with a decrease in PAC dose or an increase in NOM surface concentration. Because of the pore blockage effect of NOM, a 30% drop in atrazine removal was observed in a continuous flow PAC/microfiltration (MF) system after 7 days of contact compared to the removal predicted from the batch isotherm test. Large micropores and mesopores were found to play an important role in alleviating the effect of pore blockage. A PAC with a relatively large fraction of large micropore and mesopores was shown to suffer much less from the pore blockage effect compared with a PAC that had a much smaller fraction of large pores. Natural waters with different NOM molecular weight distribution caused different extent of pore blockage. The NOM molecules with molecular weight between 200 and 700 Dalton appeared to be responsible for the pore blockage effect.     

The application of powdered activated carbon for MIB and geosmin removal: predicting PAC doses in four raw waters

Blooms of blue-green algae in reservoirs often produce the musty-earthy taste and odour algal metabolites 2-methylisoborneol (MIB) and geosmin. MIB and geosmin are not removed by conventional water treatment and their presence in the distribution system, even at low ng L-1 levels, can result in consumer complaints. Powdered activated carbon (PAC) can effectively remove MIB and geosmin when the correct dose is applied. The homogeneous surface diffusion model (HSDM) was used to predict PAC doses required to reduce MIB and geosmin concentrations to below 10 ng L-1 at four water treatment plants in Adelaide, South Australia. In jar tests, undertaken under treatment plant conditions, the predicted doses were found to produce water of the desired quality in three of the four waters. The poor predictions found in the fourth water, which had a considerably higher turbidity, were attributed to the incorporation of PAC in a larger, denser floc, leading to a reduced effective contact time of the adsorbent. It was found that higher doses of PAC were required for both compounds to produce acceptable quality water when turbidities rose above 26 NTU.     

Characteristics of competitive adsorption between 2-methylisoborneol and natural organic matter on superfine and conventionally sized powdered activated carbons

When treating water with activated carbon, natural organic matter (NOM) is not only a target for adsorptive removal but also an inhibitory substance that reduces the removal efficiency of trace compounds, such as 2-methylisoborneol (MIB), through adsorption competition. Recently, superfine (submicron-sized) activated carbon (SPAC) was developed by wet-milling commercially available powdered activated carbon (PAC) to a smaller particle size. It was reported that SPAC has a larger NOM adsorption capacity than PAC because NOM mainly adsorbs close to the external adsorbent particle surface (shell adsorption mechanism). Thus, SPAC with its larger specific external surface area can adsorb more NOM than PAC. The effect of higher NOM uptake on the adsorptive removal of MIB has, however, not been investigated. Results of this study show that adsorption competition between NOM and MIB did not increase when NOM uptake increased due to carbon size reduction; i.e., the increased NOM uptake by SPAC did not result in a decrease in MIB adsorption capacity beyond that obtained as a result of NOM adsorption by PAC. A simple estimation method for determining the adsorbed amount of competing NOM (NOM that reduces MIB adsorption) is presented based on the simplified equivalent background compound (EBC) method. Furthermore, the mechanism of adsorption competition is discussed based on results obtained with the simplified EBC method and the shell adsorption mechanism. Competing NOM, which likely comprises a small portion of NOM, adsorbs in internal pores of activated carbon particles as MIB does, thereby reducing the MIB adsorption capacity to a similar extent regardless of adsorbent particle size. SPAC application can be advantageous because enhanced NOM removal does not translate into less effective removal of MIB. Molecular size distribution data of NOM suggest that the competing NOM has a molecular weight similar to that of the target compound.     

Removal of a synthetic organic chemical by PAC-UF systems--I: Theory and modeling

Based on the age distributions of powdered activated carbon (PAC) in reactors, the competitive bi-solute isotherm and the pore-surface diffusion mechanism, a model was developed to predict the removal of a trace synthetic organic chemical (SOC) when PAC was applied to an ultrafiltration (UF) membrane system. Independent experiments evaluated the input parameters for the model. The pore diffusion in liquid-filled pore was the dominant internal mass transfer mechanism in a PAC particle and the surface diffusion mechanism was neglected in the model. Model prediction was compared with data from pilot plant experiments treating simazine in natural water. Due to the blending and the increasing PAC in the UF loop, a transient behavior of effluent simazine concentration was observed in the UF effluent. The model predicted successfully this periodical variation of the simazine concentration in the UF loop effluent, as well as the simazine concentration in the effluent from the PAC slurry contactor. The faithful model prediction required the modeling of the phenomenon of adsorption/desorption of an SOC on membrane itself.     

Evaluation of Victorian low rank coal-based adsorbents for the removal of organic compounds from aqueous systems

Three activated carbons and two chars made from low rank coal were evaluated in terms of their ability to remove the organic compound 4-nitrophenol (4-NP) and natural organic matter (NOM) from aqueous systems. The adsorption equilibrium capacities of all adsorbents for 4-NP correlated with the micropore area of the adsorbents. Adsorption rates showed improved removal with decreasing particle size and higher carbon mass loadings. A pseudo first order model was used to fit the kinetic data, with a correlation coefficient of 0.995–0.999 for all systems.

The adsorption capacity for NOM, as measured by UV-absorbing DOC, correlated well with the pore volume and pore surface areas for pores with diameters in the range 2.7–21 nm. The trend in the adsorption capacities and removal rates of the adsorbents for NOM provided evidence that the pore size distribution is one of the most important physical characteristics of activated carbon for the adsorption of NOM.

The performance of activated low rank coal based materials was comparable to a high quality coconut-based commercial carbon in batch systems. Although the non-activated char adsorbents gave poor performance, they have potential for use in applications where poor performance can be outweighed by lower cost.     

User-oriented batch reactor solutions to the homogeneous surface diffusion model for different activated carbon dosages

This paper presents a simplified approach and user-oriented solutions to the homogeneous surface diffusion model (HSDM) equations for determining the surface diffusivity using a batch reactor system. Once the surface diffusivity is known, this model could also be used to estimate the performance of activated carbon (AC) applications as a function of contact time. In addition, fixed-bed performance can be predicted using the user-oriented solutions to the HSDM for fixed beds. The step-by-step procedure for determining surface diffusion coefficients of an activated carbon adsorber, which was initially developed by Hand, Crittenden and Thacker in 1983 for a carbon dose where C_equilibrium/C₀ = 0.5, is modified to allow calculations for different carbon dosages. This modification provides solutions to the HSDM equations for different activated carbon dosages. The solutions to the HSDM framework are provided as simplified algebraic equations suitable for quick and easy estimations of D_S. The excel spread sheet is provided in the supplemental information and a detailed example is discussed.     

Removal of a synthetic organic chemical by PAC-UF systems. II: Model application

This paper describes several application potentials with a recently developed model for predicting the synthetic organic chemical (SOC) removal by powdered activated carbon (PAC) adsorption during ultrafiltration (UF) and discusses the removal mechanism. The model was successfully applied, without any modification, to dead-end mode operation as well as to cross-flow mode operation, validating the assumption of the internal diffusion control mechanism and the continuously-stirred-tank-reactor (CSTR) concept. Even when UF was operated in a cross-flow mode, PAC added was re-circulating in suspension for only a short time. Then, solute uptake took place mostly by PAC immobilized in membrane tubes not only for dead-end operation but also for cross-flow operation. Therefore, cross-flow operation did not have any advantage regarding the SOC mass transfer on PAC in UF loop over dead-end operation. The model simulation implied that pulse PAC addition at the beginning of filtration cycle resulted better SOC removal than continuous PAC addition. However, for the pulse PAC addition mode, the model predicted somewhat lower effluent SOC concentration than the observed values, and the benefit of pulse PAC application in terms of reducing SOC over its continuous dosage was not confirmed. Longer detention time of PAC dosed in a pulse than continuously dosed PAC could possibly further decrease internal diffusivity.     

Phosphorus removal by granular activated alumina

Phosphorus removal from wastewater may be carried out by fixed-bed adsorption using activated alumina. In order to prevent unacceptable head-losses coarse-grained alumina must be used. Such systems have been referred to by several authors in literature. The mass transport characteristics of the system has so far, however, not been given a thorough investigation. This study uses the homogeneous surface diffusion model (HSDM) to describe the process as influenced by the system parameters.A sensitivity analysis is presented to optimize the process design for given conditions. The pH, the alumina particle size and the column length are found to be very important parameters determining the column performance. The process is very well suited for designing a beds-in-series system.     

Elucidating competitive adsorption mechanisms of atrazine and NOM using model compounds

Based on the relative adsorbability of natural organic matter (NOM) fractions with different molecular weights (MWs), two model compounds, poly(styrene sulfonate) (PSS) (nominal MW=1800 Dalton) and p-dichlorobenzene (DCB), were chosen to study the competitive effect of large and small NOM molecules on atrazine adsorption by two powdered activated carbons (PACs) with different pore size distributions. Both isotherm and kinetic tests of atrazine adsorption were conducted using fresh PAC and PAC preloaded with the model compounds. The model compounds were found to affect atrazine adsorption through two different mechanisms due to their size difference: direct competition for sites by p-DCB and pore constriction/blockage by PSS-1.8k. p-DCB was found to significantly reduce atrazine adsorption capacity but to have no effect on atrazine adsorption kinetics. In contrast, the effect of PSS-1.8k on atrazine adsorption capacity was very small. Furthermore, during simultaneous adsorption, PSS-1.8k had no effect on atrazine surface diffusion. However, preloading PAC with PSS-1.8k lowered the atrazine surface diffusion coefficient, D(s), by more than three orders of magnitude; D(s) decreased with increasing solid phase PSS-1.8k concentration. The pore size distribution of the PAC was found to play an important role in competitive adsorption. A high mesopore surface area could alleviate pore blockage significantly.     

Breakthrough behavior of granular ferric hydroxide (GFH) fixed-bed adsorption filters: modeling and experimental approaches

Breakthrough curves (BTC) for the adsorption of arsenate and salicylic acid onto granulated ferric hydroxide (GFH) in fixed-bed adsorbers were experimentally determined and modeled using the homogeneous surface diffusion model (HSDM). The input parameters for the HSDM, the Freundlich isotherm constants and mass transfer coefficients for film and surface diffusion, were experimentally determined. The BTC for salicylic acid revealed a shape typical for trace organic compound adsorption onto activated carbon, and model results agreed well with the experimental curves. Unlike salicylic acid, arsenate BTCs showed a non-ideal shape with a leveling off at c/c0 approximately 0.6. Model results based on the experimentally derived parameters over-predicted the point of arsenic breakthrough for all simulated curves, lab-scale or full-scale, and were unable to catch the shape of the curve. The use of a much lower surface diffusion coefficient D(S) for modeling led to an improved fit of the later stages of the BTC shape, pointing on a time-dependent D(S). The mechanism for this time dependence is still unknown. Surface precipitation was discussed as one possible removal mechanism for arsenate besides pure adsorption interfering the determination of Freundlich constants and D(S). Rapid small-scale column tests (RSSCT) proved to be a powerful experimental alternative to the modeling procedure for arsenic.     

Design and evaluation of hydraulic baffled-channel PAC contactor for taste and odor removal from drinking water supplies

Based on the concept of hydraulic flocculator, a baffled-channel powdered activated carbon (PAC) contactor, placed before the rapid-mixing basin, was designed and evaluated for removal of taste and odor (T&O) in drinking water. PAC adsorption kinetic tests for raw water samples were conducted for selection of design parameters related to contact time and degree of mixing. Within the tested range of velocity gradient (G) from 18 to 83s(-1), mixing had a relatively minor effect on the adsorption kinetics of the PAC. The hydrodynamic characteristics of the pilot-scale horizontally and vertically baffled-channel PAC contactor were investigated by tracer tests. It was found that the plug flow fractions of vertically baffled-channel PAC contactor (vBPC) were higher than those of the horizontally baffled-channel PAC contactor (hBPC) for the same bend width or bend height. However, the hBPC seems to be more appropriate than the vBPC in terms of construction and maintenance. The geosmin and MIB removal rate increased with the number of baffles, PAC dose and contact time increased regardless of bend width in the pilot-scale hBPC. The pair of full-scale hBPCs at Pohang water treatment plant, having a design capacity of 6.5x10(4)m(3)/d with 20min of hydraulic retention time with a safety factor of 2, was designed based on lab- and pilot-scale experimental results. Under a velocity gradient of 20s(-1), the number of baffles to be installed was calculated to be 20 with a space of about 2m between each baffle, resulting in a hydraulic head loss through the contactor of about 0.056m. The successful application of hBPC for T&O removal from drinking water supplies should provide momentum for developing more effective treatment methods.     

Discriminating and assessing adsorption and biodegradation removal mechanisms during granular activated carbon filtration of microcystin toxins

Microcystins are cyanobacterial toxins that are problematic for water authorities due to their resistance to conventional water treatment. Granular activated carbon (GAC) filtration has been shown to be effective in removing microcystin from water using both adsorption and biodegradation removal mechanisms; however, little is known regarding which removal mechanism predominates and to what extent. In this study, microcystin removal due to adsorption and biodegradation in GAC filtration were discriminated and assessed by commissioning three parallel laboratory columns, including a sterile GAC column, a conventional GAC column and a sand column. The results demonstrate that biodegradation is an efficient removal mechanism once it commences and that the rate of biodegradation was dependent upon temperature and initial bacterial concentration. Adsorption of microcystins was prevalent during the initial stages of the GAC columns and was modelled using the homogeneous surface diffusion model (HSDM). The HSDM provided evidence that an active biofilm present on the surface of the conventional GAC hindered adsorption of microcystin compared with the sterile GAC with no active biofilm. Up to 70% removal of microcystin-LR was still observed after 6 months of operation of the sterile GAC column, indicating that adsorption still played a vital role in the removal of this toxin.     

应对突发氯苯污染的粉末炭吸附工艺实验研究

考察了模拟常规工艺对水中氯苯的去除效能,测定了粉末炭(PAC)对原水中氯苯的吸附等温线和吸附动力学曲线,并采用Freundlich吸附等温式和假二级动力学模型进行拟合.结果表明,常规工艺难以有效去除水中氯苯;PAC可快速地吸附水中氯苯,5min吸附量可达平衡吸附量的80％以上,30 min吸附量可达98％以上.建立了P...     

Modeling the behaviors of adsorption and biodegradation in biological activated carbon filters

This investigation developed a non-steady-state numerical model to differentiate the adsorption and biodegradation quantities of a biological activated carbon (BAC) column. The mechanisms considered in this model are adsorption, biodegradation, convection and diffusion. Simulations were performed to evaluate the effects of the major parameters, the packing media size and the superficial velocity, on the adsorption and biodegradation performances for the removal of dissolved organic carbon based on dimensionless analysis. The model predictions are in agreement with the experimental data by adjusting the liquid-film mass transfer coefficient (k(bf)), which has high correlation with the Stanton number. The Freundlich isotherm constant (N(F)), together with the maximum specific substrate utilization rate (k(f)) and the diffusion coefficient (D(f)), is the most sensitive variable affecting the performance of the BAC. Decreasing the particle size results in more substrate diffusing across the biofilm, and increases the ratio of adsorption rather than biodegradation.     

NOM removal by adsorption onto granular ferric hydroxide: Equilibrium, kinetics, filter and regeneration studies

Adsorption onto granular ferric hydroxide (GFH) with subsequent in-situ regeneration is studied as a new process for natural organic matter (NOM) removal from groundwater. Adsorbent equilibrium loadings of 10-30 mgDOC g(-1)GFH(-1) are obtained, whereas the non-adsorbable DOC fraction amounts to 1.5 mgL(-1) for all investigated groundwaters. The larger and UV-active NOM fractions (mainly fulvic acids) are well adsorbed while the smaller molecular fractions are poorly or not adsorbed. However, kinetic studies show that the smaller and medium-sized fulvic acids are removed first. The equilibrium is strongly dependent on pH but only weakly on ionic strength, pointing to ligand exchange as the dominant adsorption mechanism. With regard to NOM structure, prerequisites for adsorption onto GFH are both a minimum number of functional groups and a molecular size small enough to enter the GFH pores. NOM breakthrough curves are successfully simulated using the LDF model (homogeneous surface diffusion model (HSDM) with linear driving force approach for surface diffusion) and experimentally determined mass transfer coefficients. Regeneration of loaded GFH is possible either by use of NaOH or oxidatively by H(2)O(2). The optimal quantities and concentrations are determined.