微藻采收技术的进展与展望

从微藻培养液的特性出发,总结了目前微藻采收工艺中预处理和富集分离阶段的各种方法,包括预氧化、化学絮凝、物理预处理、沉降、过滤、离心、泡载和气浮等. 已有研究结果表明,经预氧化处理后,细胞分泌胞外产物,有利于絮凝沉降,但药剂过量会引起细胞损伤甚至消亡;化学絮凝和物理处理可以大大提高后续富集分离的效率. 由于微藻培养液浓度低、粒度小,传统的沉降、过滤、离心等方法用于微藻的采收都存在效率低、成本高的限制,而气浮法适用于低浓度悬浮体系的富集分离,是微藻采收可行的技术途径之一.     

Cell and surfactant separation by column flotation

The separation of rehydrated Saccharomyces cerevisiae and an alkyl polyglycoside surfactant by column flotation was studied as a function of wash water flow and cation concentration. Separation of cells and surfactant was measured under steady-state conditions. Surfactant recovery in the foam concentrate was in the range of 86–95%. Yeast cells were enriched in the foam concentration by a factor of up to 11, but the recovery only reached 55%. The use of wash water was very effective for removing the cells from the foam, giving a good separation between the cells and the surfactant. Addition of chloride salts of Na, K, Ca, and Mg at concentrations in the range of 0.05-0.1 mol/L increased both the enrichment and the recovery of yeast in the foam. The most effective salt levels for cell flotation, less than 0.1 mol/L, were in the same range of concentration as the minimum electrophoretic mobility of the cells.     

Oil recovery from oil in water emulsions using a flotation column

A flotation column was used to recover oil from oil in water emulsions. The feed oil concentrations investigated were relatively high, in the range of 0.25 to 8 percent by volume. Previous studies using conventional flotation cells dealt with very dilute systems where the oil concentration was less than 0.1 percent by volume. The oil recovery was found to decrease with an increase in the feed oil concentration. The addition of wash water to the froth zone of the column had little effect on the oil recovery. The oil recovery decreased with increasing feed flow rate and surfactant concentration. The effect of increasing the gas flow rate was to initially increase the oil recovery. The oil recovery data were analyzed using a kinetic model. The order of the flotation kinetics was found to be 0.6. The gas hold-up behaviour of the recovery and the froth zones of the flotation column is found to correlate well with the drift-flux model.     

Diesel Oil Removal by Froth Flotation Under Low Interfacial Tension Conditions I: Foam Characteristics,and Equilibration Time

Abstract

Froth flotation is a surfactant‐based separation process which is suitable for treating dilute wastewaters. To achieve high performance for the froth flotation operation, the combination of an ultra‐low interfacial tension (IFT) between excess oil and excess water phases, high foam production rates, and high stability of the foam produced, must be attained. To obtain the ultra‐low interfacial tensions, a Winsor Type III or middle phase microemulsion has to be formed. In this study, branched alcohol propoxylate sulfate sodium salt with 14–15 carbon number and 4 PO groups (Alfoterra 145–4PO) was used to form microemulsions with diesel oil. From the results of this work, an increase in surfactant concentration decreased the IFT, and increased foam stability. To obtain the minimum IFT in the region of a Winsor Type III microemulsion, the addition of 5 wt.% NaCl was needed. However, this optimum salinity does not result in effective froth flotation due to poor foam characteristics. The results indicate that both the IFT and the foam characteristics should be optimized to achieve high efficiency of oil removal in a froth flotation operation. Unlike the previously‐studied ethylbenzene system, agitation of the solution before introduction into the flotation column yielded the lowest diesel oil removal efficiency because of the poor foam characteristics compared to either unagitated systems or systems allowed to equilibrate for one month.     

Diesel Oil Removal by Froth Flotation Under Low Interfacial Tension Conditions II: Continuous Mode of Operation

Abstract

The objective of this study was to investigate the relationship between interfacial tension (IFT) and foam characteristics and the efficiency of diesel oil removal from water in a continuous froth flotation column. The effects of operational parameters, including surfactant concentration, salinity, oil-to-water ratio, foam height, air flow rate, and hydraulic retention time (HRT) on the oil removal were investigated in the continuous mode of a froth flotation operation and compared to batch operation results. Unlike the batch system, for the continuous system used in the present study, having only branched alcohol propoxylate sulfate sodium salt surfactant (C_14–15(PO)₅SO₄Na) and NaCl present in the solution yielded such poor foam characteristics that a stable froth which overflowed the flotation column could not be produced, so the addition of sodium dodecyl sulfate (SDS) as a froth promoter was used to improve the foam stability. Unlike the batch froth flotation system with only C_14–15(PO)₅SO₄Na, the continuous froth flotation with the mixture of C_14–15(PO)₅SO₄Na and SDS, it was not possible to find a SDS and a NaCl concentration at which both ultralow IFT and good foaming were both achieved. Foam formation, stability, and production rate were found to be crucial parameters to the froth flotation efficiency. The continuous froth flotation system offers a high diesel oil removal of 96% in the single stage unit. Demonstration of efficient operation in the continuous mode in this work is important to the practical application of froth flotation in large scale processing.     

Flotation of Germanium from Dilute Solutions

Abstract

In laboratory experiments, ion flotation was found effective as a concentration method for germanium from dilute aqueous solutions. Removals over 90% were obtained at neutral pH conditions and stoichiometric addition of the flotation reagents, these being pyrogallol as activator for complex formation and laurylamine as cationic surfactant. Parameters investigated were initial and residual concentration, gas flow rate, pH of the solution, ionic strength, type of salts added, laurylamine concentration, retention time, ethanol addition, and surface tension on the germanium recovery.     

Clarification of suspensions by colloidal gas aphrons

Colloidal gas aphrons (CGAs), generated from dilute solutions of four surfactants, were used to clarify palm oil mill effluent (POME), suspensions of microalgae and suspensions of three inorganic minerals. In POME and the algal suspensions, each CGA was most effective at a pH value close to the pK value of the surfactant concerned. This effect was not tested in the inorganic suspensions. The efficiency of air utilization was directly related to the concentration of solids in the suspensions, the size, density and nature of the solids having secondary effects. Comparison with data in the literature led to a general correlation embracing a variety of suspensions and flotation systems. Shedding of collected material from the foam layer was also a direct function of solids concentration. CGAs offer advantages over other systems of air-assisted flotation in relation to the requirements for equipment and to the management of process operations.     

Biosorptive Flotation in Metal Ions Recovery

Abstract

Heavy metals can be removed from dilute aqueous solutions in many ways. Among the innovative ones may be classified a process consisting of biosorption followed by flotation. A metal cation, cadmium, was examined; the metal was abstracted by microorganisms belonging to the Actinomycetes, i.e., Streptomyces clavuligerus and Streptomyces griseus, which have a filamentous morphology, and hence present a flocculent character. Dissolved-air flotation was the technique applied on a laboratory scale without the addition of any flotation surfactant. The parameters investigated in the batch mode were contact time, recycle ratio, solution pH, Cd concentration, biomass addition, and use of a frother (ethanol). Promising results were obtained; in certain cases an almost quantitative cadmium abstraction, followed by higher than 90% biomass recovery, was found.     

Development of an efficient electroflocculation technology integrated with dispersed‐air flotation for harvesting microalgae

BACKGROUND: Various methods, including centrifugation, filtration, electroflocculation, flocculation and flotation, have been developed for harvesting microalgae. However, some economic or technical problems still remain with current methods for algal recovery, such as high capital, energy and running costs, flocculant toxicity or low separation efficiency. Therefore, there is great interest in developing new efficient approaches for harvesting microalgae. RESULT: An efficient electroflocculation method integrated with dispersed‐air flotation has been developed for harvesting Botryococcus braunii. The recovery efficiency of B. braunii reached 93.6% after 30 min using an electroflocculation process. Microalgae recovery was improved significantly when the electroflocculation process was integrated with dispersed‐air flotation; the recovery efficiency of B. braunii reached 98.9% after 14 min. CONCLUSION: The present work demonstrated that electroflocculation integrated with dispersed‐air flotation is a promising method suitable for microalgae harvesting. The strategic air supply obviously shortened the algal recovery time in the process of electroflocculation due to facilitating algal aggregation and flotation to the solution surface. Copyright © 2010 Society of Chemical Industry     

Separation of tungstates from aqueous mixtures containing impurities (arsenate,phosphate and silicate anions) using ion flotation

Ion flotation, using dodecylamine as surfactant and magnesium ions as depressant agents, was found to be an effective method for the selective separation and recovery of tungstate from arsenate anions in dilute aqueous solutions. Arsenates represent the main impurity in tungsten-containing waste waters or leaching solutions of hydrometallurgical origin; magnesium chloride is a relatively common precipitant, used in tungsten hydrometallurgy. The main parameters affecting this process were investigated, namely concentrations of reagents (Mg²⁺ ions, dodecylamine, arsenate and tungstate), solution pH, co-existence and effective separation from other impurities, such as phosphate and/or silicate anions, and the effectiveness of the separation process in solutions of high ionic strength (using NaCl and Na₂SO₄ salts). It was found that at pH range between 2 and 5, tungstate anions can be quantitatively recovered from aqueous mixtures containing arsenates, phosphates and silicates, while the co-removal of the impurities was below 20%.     

Influence of Hydrocarbon Chain Branching on Foam Properties of Olefin Sulfonate with FoamScan

The influence of surfactant structure on foam properties of internal olefin sulfonate (IOS) and alpha olefin sulfonate (AOS) in aqueous solutions was estimated from measurements of the foamability, foam stability, and foam morphology, as obtained from conductivity and image analyses techniques. It was found that the foamability and foam stability of C_16–18 AOS are higher compared to that of C_16–18 IOS, indicating that hydrocarbon chain branching decreases the foamability and foam stability. The foamability and foam stability are enhanced with increasing surfactant concentration, which increases the adsorbed quantity of surfactant molecules at the air–water interface. The influence of hydrocarbon chain branching on foam morphology was also investigated. It was found that foam cells produced by branched chain C_16–18 IOS are larger than the foam cells generated by straight chain C_16–18 AOS.     

Surface adsorption and the effect of column diameter in the continuous foam separation process

Foam separation has been investigated as a technique for removing ethylhexadecyldimethylammonium bromide, a cationic surfactant, from dilute solutions. An ideal foam model has been used to determine the surface excess accumulated at the solution-air interface and to predict the foam and drain concentration in a continuous foam separation process. It was found that the model was valid only under conditions of good drainage and perfect foam stability. An increase in column diameter for a feed of constant composition and supplied at a constant rate increased the concentration of the surfactant in the overhead stream. This highly desirable effect was accompanied by an increase in the drain rate. The results indicate that an increase in column diameter had an entirely beneficial effect on the efficiency of the separation.     

Foam Fractionation and Ion Flotation of Simple and Complex Anions with Cationic Surfactants

Experimental data are presented and interpreted on the foam fractionation of an extensive series of simple and complex anions from dilute (of the order 10⁻⁶−10⁻⁴ molar) aqueous solutions, utilizing a quaternary ammonium surfactant with which the anions form soluble ion pairs in competition with the surfactant's counterion. Selectivity coefficients, based on a bubble-interface, ion exchange model, are established in a single-equilibrium-stage, continuous-flow, foam fractionation unit for each of a series of 13 anions and oxyanions versus the surfactant's bromide counterion. Three additional series of batch experiments with multi-metal solutions establish the separation and concentration in the foam of the oxyanions of Re(VII), Mo(VI), Cr(VI), W(VI), and V(V); of the cyanide complex anions of Zn(II), Cd(II), Hg(II), and Au(III); and of the chloride complex anions of Zn(II), Cd(II), Hg(II), and Au(III). The metal oxyanions and metal cyanide and chloride complex anions can be separated from each other and from competing chloride, cyanide, or nitrate. Ion flotation of anionic species with a cationic surfactant involves an entirely different mechanism, in which a precipitation reaction occurs and particle flotation follows. Hexavalent chromium (primarily HCrO⁻₄) is ion-floated with each of a series of variable chain length quaternary ammonium surfactants, elucidating the effects of temperature, the surfactant/Cr(VI) feed ratio, mixing time, and surfactant chain length in terms of the roles of the surfactant as precipitant, dispersant, collector, and frother.     

Optimization of flocculation with tannin-based flocculant in the water reuse and lipidic production for the cultivation of Acutodesmus obliquus

This paper evaluated the optimization of natural flocculant in different conditions in a tubular photobioreactor using swine wastewater effluent as culture media in a pilot-scale microalgae production plant. This study aimed to assess the flocculation efficiency of Tanfloc SG via a central composite design (CCD), with varying Tanfloc SG concentration, biomass concentration and pH. Subsequently, the microalga Acutodesmus obliquus was cultivated in medium obtained from the recycled recovery of microalgae cells via flocculation with Tanfloc SG. Results showed that the recycled medium after flocculation with Tanfloc SG had a positive effect on the biomass and lipid production of A. obliquus.     

Recovery of Surfactant from an Aqueous Solution Using Continuous Multistage Foam Fractionation: Mixed Surfactant System

A continuous multistage foam fractionation column with bubble caps was used for surfactant recovery from mixed surfactant solutions containing polyethylene glycol tert-octylphenyl (OPEO₁₀) and cetylpyridinium chloride (CPC) and the effects of air flow rate, foam height, and feed flow rate were investigated under a steady state of conditions. For the mixed surfactant system, the effect of synergism in the surfactant adsorption density was found. For separation efficiency, the total residual factor remained unchanged with an increasing feed molar fraction of OPEO₁₀ (α), suggesting that the addition of OPEO₁₀ does not increase the total separation efficiency. The residual factor of CPC increased with an increasing molar fraction of OPEO₁₀ (α), while the residual factor of OPEO₁₀ was lower for the mixed surfactant systems. A competitive removal was found in that the OPEO₁₀ can compete with CPC for the bubble surface. The total separation factors and enrichment ratio of mixed surfactant systems were in-between the two single surfactant systems at a long foam residence time and, in contrast, showed antagonism at short foam residence. This is due to the difference in liquid entrainment in foam at long and short foam residence times.     

Powdered Activated Carbon Separation from Water by Foam Flotation

Abstract

Powdered activated carbon was separated from dilute aqueous suspensions (200–1000 mg/L) by foam flotation using surfactants (anionic or cationic). The effects of surfactant type, pH value of the suspension, initial carbon and surfactant concentrations, flotation time, and air flow rate on the dispersed-air flotation of powdered activation carbon were investigated. In optimum conditions the powdered activated carbon separation was almost complete. The ζ-potential of powdered activated carbon was also measured in the presence and absence of surfactants. Finally, carbon flotation was examined after the carbon had adsorbed chromate ions from an acidic solution (pH 2). Almost complete separation of Cr(VI)-loaded carbon was obtained by using an anionic surfactant.     

Foam separation of active carbon

An experimental investigation is presented of the foam separation of powdered active carbon, equilibriated with an aqueous, synthetic waste water containing phenol and a cationic (ethylhexadecyldimethylammonium bromide (EHDA-Br)), anionic (dodecyl sodium sulphate), or non-ionic (alkyl phenoxy polyethoxy ethanol) surfactant. The effect of surfactant, of pH, of initial carbon concentration, and of initial surfactant concentration on the flotation of carbon is investigated. At pH 3, 7, and 10, the cationic surfactant yields the best flotation of carbon, which increases with increasing pH. At pH 7, a suspension containing 800 mg/1 carbon can be reduced to 24 mg/1 in 10 min. with 0·37 mM EHDA-Br. The relative concentrations of carbon and of surfactant must be controlled carefully to yield sufficient free surfactant to obtain a foam but not excessive free surfactant to impair the foam separation process. Foam volumes are controlled by free (non-adsorbed) surfactant.     

A New Hybrid Flotation—Microfiltration Cell

Abstract

The investigated hybrid cell combines the advantages of both flotation and membrane separation, while overcoming their limitations and having as an outcome clean water from a industrial wastewater. Hence, metals recovery from dilute aqueous solutions was a promising application of this innovative process, further to solid/liquid separation. The specific objective was to apply the process for the efficient separation of effluents containing metals (here, zinc). The main examined parameters were the following: the metal initial concentration, flotation surfactant applied, and air flowrate. The successful contribution of precipitate flotation was highlighted, while the observed metal removals were of the order of ～100%.     

Removal of mercury by foam fractionation using surfactin, a biosurfactant

The separation of mercury ions from artificially contaminated water by the foam fractionation process using a biosurfactant (surfactin) and chemical surfactants (SDS and Tween-80) was investigated in this study. Parameters such as surfactant and mercury concentration, pH, foam volume, and digestion time were varied and their effects on the efficiency of mercury removal were investigated. The recovery efficiency of mercury ions was highly sensitive to the concentration of the surfactant. The highest mercury ion recovery by surfactin was obtained using a surfactin concentration of 10 × CMC, while recovery using SDS required < 10 × CMC and Tween-80 >10 × CMC. However, the enrichment of mercury ions in the foam was superior with surfactin, the mercury enrichment value corresponding to the highest metal recovery (10.4%) by surfactin being 1.53. Dilute solutions (2-mg L(-1) Hg(2+)) resulted in better separation (36.4%), while concentrated solutions (100 mg L(-1)) enabled only a 2.3% recovery using surfactin. An increase in the digestion time of the metal solution with surfactin yielded better separation as compared with a freshly-prepared solution, and an increase in the airflow rate increased bubble production, resulting in higher metal recovery but low enrichment. Basic solutions yielded higher mercury separation as compared with acidic solutions due to the precipitation of surfactin under acidic conditions.     

Experimental investigation of foam performance in the presence of crude oil

The foam performance in the presence of oil plays an important role in foam application in enhancing oil recovery. The present study systematically investigated the effect of oil type, oil content, surfactant type, surfactant concentration, alkane chain length, salinity, and polymer concentration on foam performance in both the absence and presence of oil. The results showed that oil viscosity and oil density as well as oil component all contributed to foam performance in the presence of oil. Within a certain oil content, both light oil and heavy oil had a positive effect on foam, but heavy oil had a higher tendency to stabilize the foam. The order of foam performance by different surfactants was changed by the oil. It is noteworthy that heavy oil is detrimental to sodium dibutyl naphthalene sulfonate (BM) foam. Light oil can improve foam performance while heavy oil can harm foam in some specific cases. Lower salinity, longer alkane chain length, higher surfactant concentration and the presence of a polymer all benefited foam in the presence of crude oil.