Separation of Humic Acids from Bayer Process Liquor by Membrane Filtration∗

Abstract

Humic acids of high molecular weight were removed from spent Bayer liquor by polymeric ultrafiltration membranes. Among the commercial and laboratory-cast membranes tested, Radel-R polyphenylsulfone on a polypropylene backing material was found to be the most promising candidate for this separation. However, the maximum separation of humic acids obtained at operating conditions of 50°C and 0.34 MPa, as measured by spectrophotometric analysis, was only in the 50 to 55% range. In order to explain this limited membrane separation of humic acids in spent Bayer liquor, a synthetic alkaline solution of humic acids was treated using the same membranes. These tests indicated much higher separation of humic acids (92%). Humic substances in Bayer liquor appear to be hydrolyzed and degraded to low molecular weight fractions (molecular weight < 1000 daltons) by the combined action of the strongly alkaline Bayer liquor and high digestion temperatures. These low molecular weight fractions cannot be retained by standard ultrafiltration membranes. However, some preliminary tests with laboratory-cast Radel-R nanofiltration membranes showed improved color separation (>70%) when treating spent Bayer liquor. Characterization of these new membranes, at different temperatures and pressures as well as by other factors influencing separation of humic acids from Bayer liquor, were investigated.

     

High performance gel permeation chromatography of polystyrene

Microparticulate crosslinked polystyrene packings in short columns have been investigated with high performance liquid chromatography instrumentation. Reliable molecular weight data for six polystyrene standards having narrow molecular weight distributions and for a polystyrene having a broad distribution have been obtained by optimizing the injection procedure, using a constant flow pump, and incorporating an internal standard into each injected solution. Experimental determinations of the dependence of the polydispersity for polystyrene standards on eluent flow rate and polymer diffusion coefficient were in agreement with a relation predicted from theoretical considerations of chromotogram broadening. Because of the dependence of chromatogram broadening on polystyrene molecular weight, high efficiency separations for high polymers were only obtained at low eluent flow rates. For low polymers, high efficiency separations may be performed at fast eluent flow rates. It was concluded that accurate molecular weight distributions can only be determined from chromatograms obtained at low eluent flow rates, which was supported by experimental measurements of polydispersity on polystyrene sample prepared by a radical polymerization at low monomer conversion. A differential weight distribution calculated from an experimental chromatogram for the polydisperse polystyrene determined at the lowest eluent flow rate (0.1 cm³min^?1) was compared with distributions predicted theoretically for polystyrenes prepared by radical polymerization. It was concluded that the experimental distribution contained a small contribution from chromatogram broadening and that most of the radicals in the polymerization of styrene terminated by combination.     

Exact Analysis of Field-Flow Fractionation

Abstract

A rigorous convective diffusion theory is formulated for the predictive modeling of field-flow fractionation (FFF) columns used for the separation of colloidal mixtures. The theory is developed for simulating the behavior of a colloid introduced into fluid in time-dependent flow in a parallel plate channel across which a transverse field is applied. The methodology of generalized dispersion theory is used to solve the model equations. The theoretical results show that the cross-sectional average concentration of the colloid satisfies a dispersion equation with time-dependent coefficients. The results of this work, in principle, are valid for all values of time since the introduction of the colloid. It is shown that these results asymptotically approach those of the nonequilibrium theory formulated by Giddings for large values of time.

Illustrative numerical results are obtained for the case of steady laminar flow and a uniform initial distribution. The behavior of the coefficients in the dispersion equation is explained on physical grounds. Of particular interest is the fact that at large values of the transverse Peclet number P, Taylor dispersion in the FFF column is very small. Under these conditions, axial molecular diffusion as well as Taylor dispersion in the connecting tubing could make a substantial contribution to the axial dispersion observed in practical FFF columns.

The theoretical predictions are compared with the experimental data of Caldwell et al. and Kesner et al. on electrical FFF columns. The comparisons indicate that the theory has potential in predicting the performance of such systems.     

Thermogravitational Field-Flow Fractionation: An Elution Thermogravitational Column

Abstract

The major operating characteristics of thermal field-flow fractionation (thermal FFF) and of thermogravitational columns are compared, and it is shown that the two approaches can be advantageously combined in a method we call thermogravitational FFF. The theory of this technique is developed, with primary attention given to a change in the velocity profile under different flow conditions and its effect on component retention, column efficiency, resolution, and selectivity. Experimental results are shown to be in good overall accord with theory. It is shown that the potential of thermogravitational FFF lies in the fractionation of low molecular weight polymers or of other species having weak thermal diffusion.     

Parameters Affecting Magnetic Field-Flow Fractionation of Metal Oxide Particles

Abstract

Magnetic field-flow fractionation (FFF) is a new separation technique particularly suited to separations involving particulate materials of high magnetic permeability. In this technique a magnetic field, generated by an electromagnet, is used to induce retention of particles in the FFF flow stream. This paper discusses the theoretical basis for magnetic FFF in terms of the fundamental retention equation of FFF and the magnetic force equation. Experimental data are presented which characterizes the magnetic field and the retention process. The resolution of single particles from dimeric aggregates is demonstrated based upon their difference in volume.     

Molecular dynamics simulation of diffusion of hydrogen and its isotopic molecule in polystyrene

Polystyrene is one of the target materials used in Inertial Confinement Fusion (ICF). Molecular dynamics simulations are performed in this report to study the diffusion of gases, including hydrogen and its isotopic molecule under normal temperature and pressure. According to the Mean Square Displacement (MSD) of the gas moving in polystyrene, the diffusion coefficient of hydrogen, deuterium and tritium in different molecular weight polystyrene were obtained. The calculated diffusion coefficients agree well with the results of former simulation studies. The diffusion coefficients of polystyrene of the same molecular weight gradually decrease along with the increase of mass fraction of hydrogen isotopes (hydrogen, deuterium and tritium). The study also finds that diffusion coefficients will decrease along with the increasing of polystyrene molecular weight. Moreover, the pair correlation functions, cohesive energy density and fractional free volume were studied corresponding to hydrogen isotopes diffusion coefficients.     

Effects of Ozonation on Molecular Weight Distribution of Humic Substances and Coagulation Processes – A Case Study: The Úzquiza Reservoir Water

In this article the influence of preozonation on the effectiveness of NOM removal via coagulation processes will be studied (focusing on the influence of the calcium hardness) as well as changes in MW (molecular weight) distribution of humic substances caused by ozonation. Additionally, THMFP removal in both ozonation and preozonation-coagulation processes is assessed. Three different types of water have been used in this study: a natural water from the Úzquiza Reservoir (Burgos, Spain), a synthetic water prepared using natural fulvic acids extracted from the Úzquiza Reservoir and a synthetic water prepared using a commercially supplied humic acid. Molecular weights of humic substances were determined using high-performance size exclusion chromatography (HPSEC); average molecular weights calculated for the unozonated humic substances are 4500 Da for the commercial humic acids and 1000 Da for the natural fulvic acids extracted from the Úzquiza Reservoir. Preozonation shifted the molecular weight distribution of humic substances (both humic and fulvic acids) towards lower average molecular weight values. For the natural water from the Úzquiza Reservoir (with low levels of calcium hardness and hydrophobic fraction (humic substances) being the main fraction of NOM), preozonation has a negative effect on the effectiveness of the coagulation process for NOM removal: the percentages of TOC removal via coagulation decrease with increasing ozone dosage; the maximum TOC removal (33%) is achieved for the unozonated water. Also for this water, ozonation reduced 5–25% of THMFP with ozone doses varying from 0.25 to 2.5 mg O₃/L. A preferential THMFP removal, that is to say, higher reduction in THMFP (43%) relative to TOC (28%) is achieved by the coagulation-flocculation process; this also occurs when preozonation is used, independently of ozone dosage.     

Adsorption mechanism of humic and fulvic acid onto Mg/Al layered double hydroxides

The adsorption of humic and fulvic acids onto nitrate, chloride and carbonate intercalated layered double hydroxides with Mg/Al ratios ranging from 85/15 to 60/40 was studied by adsorption isotherms at different ionic strengths and the characterization of the preferentially adsorbed humic substance size fractions. The adsorption of humic and fulvic acids onto LDHs occurred by ion exchange with both the intercalated and surface anions of the LDH and ligand exchange reactions with surface groups. The contribution of both mechanisms to the total HA and FA adsorption was estimated. Lower molecular weight humic and fulvic acids were preferentially adsorbed because these fractions can more easily enter the mesoporous LDHs and contain more carboxylic groups, which are known to be involved in ligand exchange reactions with e.g. surface Al-OH groups. Intercalation of the entire HA or FA molecules in-between the LDH sheets is unlikely to occur.     

Crossflow Microfiltration-Fouling Mechanisms Studies

Abstract

Our objective has been to improve the commercial potential of a crossflow microfiltration process with lime softening of surface waters rich in humic and fulvic acids. These components complicate the filtration of CaCO₃ solids both in the filter cake layer and through direct interactions with the nylon membrane support surface. We characterized the humic/fulvic acid fouling and developed several strategies for alleviating the problem. Physical cleaning methods like crossflow and backflush alone are ineffective. We devised chemical pretreatment and chemical cleaning procedures which improve performance.     

Effects of Ozonation on Natural Organic Matter Reactivity in Adsorption and Biodegradation Processes – A Case Study: The Úzquiza Reservoir Water

In this paper the influences of preozonation on the effectiveness of NOM adsorption and biodegradation processes are studied. Three different types of water have been used in this study: A natural water from the Úzquiza Reservoir (Burgos, Spain), synthetics waters prepared using natural humic substances (fulvic and humic acids extracted from the Úzquiza Reservoir) and a synthetic water prepared using a commercially supplied humic acid. The effect of preozonation on NOM adsorption by activated carbon is evaluated: adsorption of humic acids (hydrophobic, high molecular weight compounds) is improved following preozonation; however, for the fulvic acids (hydrophilic, low molecular weight compounds), no net appreciable effect of preozonation on adsorption was observed. Preozonation increases the biodegradability of NOM: biodegradable dissolved organic carbon (BDOC), which was determined using two different bioassays (Billen-Servais method and Joret-Lévi method), increases with increasing ozone dosage. A characterization of humic substances based on their adsorption and biodegradability properties is also reported, showing the effect of ozone.     

Measurement of polydispersity of ultra-narrow polymer fractions by thermal field-flow fractionation

In this paper we demonstrate that the polydispersity µ = M?_w/M?_N of narrow polymer fractions can be readily obtained by measuring band broadening and its velocity dependence in a thermal field–flow fractionation (thermal FFF) system. The thermal FFF method is shown to be more accurate than size exclusion chromatography for the determination of polymer polydispersities due to the simpler band dispersion function and the higher selectivity inherent to the technique. The polydispersities of a series of four narrow polystyrene samples prepared by anionic polymerization were consequently determined by thermal FFF and found to be much smaller (1.003–1.006) than the ceiling values (1.06) suggested by the suppliers. As part of this investigation, an experimental study of band dispersion in thermal FFF is used to examine current theory. The data show nonequilibrium to be the dominant factor, whereas relaxation effects are insignificant at lower flow rates and can be subdued at higher flow rates. A high correlation between nonequilibrium theory and experiment allows for the estimation of diffusion coefficients from plate height–velocity data.     

腐植酸与普洱茶茶色素

对云南普洱茶特有品质与化学组成(多酚类、儿茶素、黄酮类、茶黄素、茶红素、茶褐素)的关系进行简要地阐述。茶色素被证实是一类具有多种生理活性的物质。依据腐植酸的形成及化学性质、结构特征,提出了普洱茶中茶色素与腐植酸,尤其是茶褐素和黄腐酸,在化学性质方面(结构和官能团)存在一定的相似性。由此提出了腐植酸(黄腐酸)在生理活性及治疗功能方面与茶色素应具有相似性。为腐植酸(黄腐酸)在医药方面的应用研究提出了新思路。     

Changes in the composition of humic substances depending on the depth of peat occurrence

The fractional group composition of humic substances and the chemical structure of humic acids isolated from peats of different genesis from different depths of occurrence in peat deposits with the use of two extractants (0.1 N NaOH and 0.1 M Na₄P₂O₇ at pH 7) were studied. Peat from the upper aerated layer of the peat deposits contained more humic substances than peat from the deep layer, which occurred under anaerobic conditions, due to the pyrophosphate fraction of humic acids and fulvic acids. The humic acids of peat isolated from the deep layers of deposits had smaller molecular weights, and they were characterized by lower concentrations of carbohydrate and saturated hydrocarbon fragments and an increased contribution of aromatic polyconjugation systems, as compared with the humic acids of a surface layer.     

Shear Field-Flow Fractionation: Theoretical Basis of a New,Highly Selective Technique

Abstract

Shear field-flow fractionation (shear FFF) is described as an FFF system in which shear forces are responsible for migration perpendicular to flow. It is shown that a desirable configuration for shear FFF is a concentric cylinder system with one cylinder rotating. After providing the relevant theoretical framework of FFF, the equations of Shafer et al. describing shear migration are simplified and applied to the limiting case of very thin annular spaces to get tractable retention expressions. On this basis the maximum selectivity is predicted to be 3 or greater, a value considerably higher than that for any other macromolecular separation technique. This high selectivity is confirmed using an alternate shear migration theory developed by Tirrell et al. However, it is shown that shear FFF is only applicable to macromolecules of high molecular weight, perhaps ～10⁷ and above. It may also be applicable to globular particles.     

Simulation of nanotube separation in field-flow fractionation (FFF)

A Brownian dynamics simulation based on a prolate spheroid particle model has been developed to model the separation of nanotubes in cross flow driven, field-flow fractionation (FFF). The particle motions are governed by stochastic forms of a linear momentum balance with orientation dependent drag and diffusion coefficients, and the Jeffrey equation with rotational diffusion. The simulation shows that nanotube scale particles would be expected to elute by a normal mode mechanism up to aspect ratios of about 1000, based on a particle diameter of 1 nm. Separation of nanotubes of different length is governed by the value of the retention variable for each component in agreement with theory. Elution profiles and average velocity through the device as a function of particle size, and the flow rates in the throughput and cross-flow directions are examined. The simulation shows that clean separations between components of different size is achieved when the ratio of the retention values is greater than 2.     

Field-Flow Fractionation

Abstract

Field-flow fractionation (FFF) is a separation method first described in 1966 (I). FFF is an elution technique, like chromatography, and the experimental sequence of pump, column, detector, and fraction collector is much like that used in chromatographic operations (2-4). However, FFF appears to be unique in its ability to separate an extremely broad range of molecules, macromolecules, supramacromolecular structures, colloidal particles, and larger particles at a high level of resolution. In dealing with these complex and often refractory materials, FFF has a number of unique advantages (5). Along with its intrinsically high resolving power, FFF is a versatile technique in which experimental conditions can be varied widely to optimize the range, speed, and power of the separation. FFF is also unusual in that the characteristics of the separation can be calculated rather exactly in terms of well-defined physicochemical parameters such as molecular weight, size, charge, etc. The equations used for this purpose can be inverted to yield molecular weight and other important parameters for the components of complex mixtures (3-6).     

Flow injection analysis estimation of diffusion coefficients of pauci- and polydisperse polymers such as polystyrene sulfonates

Flow injection analysis, often used for determination of diffusion coefficients of nonpolymeric substances, has now been applied to the characterization of pauci- and polydisperse polymers in solution. A relative method was found useful for obtaining moderate quality evaluations of diffusion coefficients and related parameters of polymers. The width at half-height W_1/2 of the trace peak is found to be proportional to the number average molecular weight M?_n of pauci- and polydisperse polymers, allowing estimation of M?_r and diffusion coefficients. For sodium polystyrene sulfonates at substantially infinite dilution in 1.0 g L^?1 Na₂SO₄, a linear relation has been observed between the logarithms of the molecular weight M?_n and the mean diffusion coefficient D in the M?_n range of 1000–90,000 g mol^?1 or the D range of 30 × 10^?7 to 2 × 10^?7 cm² s^?1.     

Universal calibration for thermal field-flow fractionation

Experiments have shown that the thermal diffusion coefficient of a polymer is independent of the sample's molecular weight. According to the relationship between the ordinary diffusion coefficient and polymer molecular weight, a universal calibration method for thermal field-flow fractionation (FFF) has been created. This method has been examined experimentally by three polymers of different chemical composition with different molecular weight in three organic solvents. It is shown that this method is useful both in terms of calibrating a thermal FFF system with some readily available polymer standard, for use with unknown samples of known thermal diffusivity, and in terms of determining the thermal diffusion coefficient of the unknown sample under conditions where its molecular weight can be measured by other methods.     

Flow-modified permittivity of solutions of poly(n-butyl isocyanate) and poly(n-hexyl isocyanate)

The dielectric behaviour of solutions of poly(n-butyl isocyanate) and poly(n-hexyl isocyanate) in flow have been examined, including, for the latter polymer, fractions of differing molecular weight. The data are analysed in terms of established theories for changes in the permittivity of solutions of rigid macromolecules undergoing shear and are critically compared to dielectrically based information obtained from studies of still solutions. Rotational diffusion coefficients and dipole moments have been measured and the influence of heterogeneity in molecular weight on flow-modified permittivity is examined. The results obtained support the view that the theories for the flow-modified permittivity of rigid macromolecules are generally satisfactory in providing rotational diffusion coefficients and resolved dipoles, but that the influence of molecular shape on the effect is not simply obtained when polydisperse material is involved.     

Clusius-Dickel Separation: A New Look at an Old Technique

Abstract

The Clusius-Dickel (CD) technique couples thermal diffusion to a counter-current, natural convective flow to effect separation of molecules of different molecular weights or shapes in liquid or gaseous solutions. Materials separable by the technique include: aqueous biological solutions, isotopes in both gas and liquid states, aqueous solutions of both ionized and un-ionized materials, organic solutions, liquid metallic solutions, fused salt solutions, and gas mixtures including ortho and para-hydrogen.

The technique was discovered in the late 1930s by K. Clusius and G. Dickel but fell into relative obscurity for commerical applications about 20 years after its discovery. The reasons for its decline included a number of deleterious apparatus and operating features among which were large power requirements, costly equipment construction, and small processing volumes. The present study, however, after critically reexamining the Clusius-Dickel separation (CDS) technique, concludes that the objectionable features of the technique can probably be circumvented by a number of means among which are: utilizing modified cell designs; combining the CD technique with other separative techniques or principles, i.e., electrophoresis, field flow fractionation (FFF), adsorption (parametric pumping), or chemical reactions; and conducting the CD operation in space environments. The first two means would benefit still further from space environments. It is also concluded that the potential of the technique for separations of biological and other aqueous solutions has been largely overlooked. Furthermore, the CD principle appears to offer novel application such as solar water desalination, enery conversion devices, and a use for waste heat.

In addition to presenting the mentioned critical reexamination and protential applications, the present document reviews the history and theoretical basis of the technique.