Removal of trace hexavalent chromium from aqueous solutions by ion foam fractionation

Liquid foams: Properties, structures, prevailing phenomena and their applications in chemical/biochemical processes

Liquid foams are gas-liquid dispersions with flexible structures that provide high gas-liquid interfaces. This property nominates liquid foams as excellent gas-liquid contactors, systems that are widely used in the chemical and biochemical industries. However, challenges such as a lack of comprehensive understanding and foam instability have historically hindered their widespread industrial use in most applications. It was not until the recent development of nanofluidics, nanotechnology, surface science, and other related fields that the understanding, analysis, and control of foam phenomena improved. This led to the development of innovative stabilization techniques and foam-based unit operations in chemical and biochemical processes, each of which requires in-depth and exclusive reviews to fully comprehend their potential and limitations and to identify areas for further improvement and innovation. This paper reviews the foams, the common phenomena in them, the characteristics that make them suitable for chemical/biochemical engineering, reports on their current applications and recent developments in this field.

Construction of UiO-66-NH2 decorated by MoS2 QDs as photocatalyst for rapid and effective visible-light driven Cr(VI) reduction

The photoactive metal-organic frameworks (MOFs) are good candidates for photocatalysts, but the quick electron-hole pairs recombination has greatly restricted the photocatalytic ability of MOFs. To improve the photoactivity of MOFs, MOFs-based composite materials have been extensively studied. Here, we successfully integrated MoS2 quantum dots (QDs) with UiO-66-NH2 for the first time under hydrothermal conditions. The as-prepared MoS2 QDs/UiO-66-NH2 (MS-U) had good visible light response ability (absorption edge at 445 nm), and charge separation and transfer ability, which lays the foundation for the photocatalytic Cr(VI) reduction. Photocatalytic studies revealed that MoS2 QDs-5/UiO-66-NH2 (MS-U-5) had superior Cr(VI) reduction activity than pure MoS2 QDs and UiO-66-NH2. MS-U-5 could remove 98% Cr(VI) at pH= 2 with visible light irradiation for 20 min, which is the fastest visible light driven Cr(VI) reduction rate among the reported MOFs-based composite photocatalysts without the presence of any cocatalysts or scavengers as far as we know. Importantly, MS-U-5 could be reused at least three times. In the end, the possible electron transfer path and mechanism of Cr(VI) reduction was also investigated.

Model for Investigating Relationships between Surfactant Micropollutant Properties and Their Separation from Liquid in a Bubble Column

The removal of surfactant micropollutants, such as dyes, pharmaceuticals, and proteins, through foam is very important in biotechnology and wastewater treatment. The literature shows that previous models consider mass balances within the foam but not the adsorption dynamics of micropollutant surfactants on bubble surfaces in the liquid solution. Thus, the main objective of this work is to examine the removal of surfactant micropollutants in a bubble column considering both mass balance and adsorption dynamics to calculate surfactant transport from the liquid bulk to the bubble surface. This allows investigation of the relationships between surfactant hydrophobicity and surfactant separation efficiency from the liquid. It was found that the removal of the surfactant strongly depends on the dynamic adsorption behavior of surfactant on bubble surfaces, and the highest foam fractionation performance was achieved when the surfactant molecule was highly hydrophobic. This work demonstrates that the adsorption dynamics rather than adsorption thermodynamics on bubble surfaces is critical when modeling the removal of surfactant micropollutants from water solutions.

Effect of different co-foaming agents on PFAS removal from the environment by foam fractionation

Per- and poly-fluoroalkyl substances (PFAS) are recalcitrant, synthetic chemicals that are ubiquitous in the environment because of their widespread use in a variety of consumer and industrial products. PFAS contamination has become an increasing issue in recent years, which needs to be urgently addressed. Foam fractionation is emerging as a potential remediation option that removes PFAS by adsorption to the surface of rising air bubbles which are removed from the system as a foam. PFAS concentrations in the environment are often not sufficient to allow for formation of a foam by itself and often a co-foaming agent is required to be added to enhance the foamability of the solution. In this study, the effect of different classes of co-foaming agents, anionic, non-ionic, zwitterionic and cationic surfactants on the removal of PFAS with varying fluorocarbon chain length from 3 to 8 in a foam fractionation process have been investigated. Evaluation of the air-water interface partitioning coefficient (k') in addition with surface tension and PFAS removal results support the contention that using a co-foaming agent with the opposite charge to the PFAS in question significantly facilitates the adsorption of PFAS to the air-water interface, enhancing the efficiency of the process. Using the non-ionic surfactant (no headgroup electrostatic interaction with PFAS), as a reference, it was observed, in terms of PFAS separation and rate of PFAS removal, that anionic co-surfactant performed worst, zwitterionic was better, and cationic co-surfactant performed best. All of the PFAS species were able to be removed below the limit of detection (0.05 µg/L) after 45 minutes of foaming time with the cationic surfactant.

Separation and Purification of Recombinant β-Glucosidase with Hydrophobicity and Thermally Responsive Property from Cell Lysis Solution by Foam Separation and Further Purification

The aim of this study was to separate and purify recombinant β-glucosidase (GLEGB) with elastin-like polypeptide (ELP) and graphene-binding peptide (GB) from cell lysis solution by foam separation and further purification. The study of foam property of GLEGB cell lysis solution indicated that it had excellent foaming property and foam stability, which was suitable for foam separation. This could be due to the GB tag with hydrophobicity, which made the recombinant β-glucosidase with GB preferentially adsorb on the surface of bubbles. At optimum operating conditions of foam separation, the enzyme activity recovery of GLEGB could reach 95.63 ± 1.0%. The foam solution of GLEGB was further purified based on the thermally responsive property of the ELP tag, and the purification fold of GLEGB could reach 29.6 ± 0.5 at the optimum operating conditions. The prominent purification effect indicates that this technique is a simple and efficient technique for the separation and purification of recombinant enzymes.

Preferential Removal of Alkali Metal Using Dodecanoic Acid and Sodium Dodecyl Sulfate in Foam Separation System

The selectivity of adsorption between alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) based on the ionic functional groups of the surfactants was studied using two types of surfactants, dodecanoic acid (DA) and sodium dodecyl sulfate (SDS), in the foam separation system. The results showed that Li⁺ was preferably removed by foam separation using DA. The removal rates of other alkali metal ions were relatively low, and there were no significant differences among other alkali metal ions (Na⁺, K⁺, Rb⁺, and Cs⁺). However, Cs⁺ exhibited the highest removal rate among the mixed alkali metals by foam separation using SDS. From these results, the selectivity of the alkali metal in foam separation was dependent on the type of surfactant.

Simultaneous separation of glycyrrhizic acid, baicalein and wogonin from Radix Glycyrrhizae and Radix Scutellariae using foam fractionation and in vitro activity evaluation

BACKGROUND

In this study, the optimal conditions for the extraction and purification of glycyrrhizic acid from Radix Glycyrrhizae (RG) and baicalein and wogonin from Radix Scutellariae (RS) by foam fractionation were studied on the basis of central composite design (CCD) and response surface methodology.

RESULTS

The results showed that herbal proportion (RG:RS), gas flow and ethanol concentration were the main factors guiding the foam fractionation of RG and RS. The optimum technological parameters were obtained as follows: herbal proportion (RG:RS), 1.86:1.14; gas flow, 109 mL min^-1 ; and ethanol concentration, 53%. Under the optimal operating conditions, the maximal extraction yields of baicalein, glycyrrhizic acid and wogonin were 56.67, 13.25 and 9.51 mg g^-1 , respectively, which were 2.32-, 1.22- and 1.84-fold higher than those of ultrasonic extraction and 17.28-, 1.15- and 9.91-fold higher than those of ultrasonic extraction without hydrolysis, respectively. Investigations on the antioxidant activity showed that the foam-fractionated extract exhibited better free radical scavenging activity (IC₅₀ 13.80 μg mL^-1 ) than that of the ultrasonic extract (IC₅₀ 223.00 μg mL^-1 ). Antibacterial activity showed that the minimum inhibitory concentrations of the foam fractionated extract against Staphylococcus aureus, Candida albicans, Group A Streptococcus and Pseudomonas aeruginosa were 1.38, 1.38, 0.69 and 5.50 mg mL^-1 , respectively.

CONCLUSION

The results indicate that the foam fractionated extract exhibited better extraction yields and free radical scavenging activity than did the ultrasonic extract. Therefore, this fast and eco-friendly method was established and could be a basis for the extraction and separation of other active constituents from herbal medicines. © 2022 Society of Chemical Industry.

Efficient treatment of aged landfill leachate containing high ammonia nitrogen concentration using dynamic wave stripping: Insights into influencing factors and kinetic mechanism

Aged landfill leachate is challenging to treat owing to its extremely high ammonia concentration and poor biodegradability. We constructed pilot-scale dynamic wave stripping equipment to separate ammonia from landfill leachate and achieved excellent results. To further expand the usage of pilot-scale equipment in actual water treatment process and implement it in a sewage plant, we established the mass transfer kinetic physics and mathematical model of the dynamic wave stripping process based on the surface renewal theory and the traditional stripping method. The surface renewal theory and the traditional stripping method are employed to analyze the mechanism of various experimental parameters affecting the stripping process, predict the stripping effect of the equipment under different conditions, and verify the calculation results of the model using the kinetic fitting results of the experimental data. These calculation results of the model indicate that the mass transfer kinetic coefficients of ammonia stripping at 20 °C, 25 °C, and 30 °C are 85.62 min, 75.34 min, and 65.88 min, respectively, when the gas-liquid ratio is 129. When the gas-liquid ratios are 62, 129, and 163 at 25 °C, the mass transfer kinetic coefficients of ammonia stripping are 102.61 min, 75.34 min, and 61.43 min, respectively. With increasing temperature and gas-liquid ratio, the particle size and number of bubbles in the wave tube of the stripping equipment gradually decrease and the mass transfer efficiency of free ammonia between the gas and liquid phases improves, enhancing the stripping efficiency of ammonia nitrogen.

Process enhancing strategies for the reduction of Cr(VI) to Cr(III) via photocatalytic pathway

This discourse aimed at providing insight into the strategies that can be adopted to boost the process of photoreduction of Cr(VI) to Cr(III). Cr(VI) is amongst the highly detestable pollutants; thus, its removal or reduction to an innocuous and more tolerable Cr(III) has been the focus. The high promise of photocatalysis hinged on the sustainability, low cost, simplicity, and zero sludge generation. Consequently, the present dissertation provided a comprehensive review of the process enhancement procedures that have been reported for the photoreduction of Cr(VI) to Cr(III). Premised on the findings from experimental studies on Cr(VI) reductions, the factors that enhanced the process were identified, dilated, and interrogated. While the salient reaction conditions for the process optimization include the degree of ionization of reacting medium, available photogenerated electrons, reactor ambience, type of semiconductors, surface area of semiconductor, hole scavengers, quantum efficiency, and competing reactions, the relevant process variables are photocatalyst dosage, initial Cr(VI) concentration, interfering ion, and organic load. In addition, the practicability of photoreduction of Cr(VI) to Cr(III) was explored according to the potential for photocatalyst recovery, reactivation, and reuse reaction conditions and the process variables.

Evaluation of an Ionic Porous Organic Polymer for Water Remediation

The targeted synthesis of a novel ionic porous organic polymer (iPOP) was reported. The compound (denoted as QUST-iPOP-1) was built up through a quaternization reaction of tris(4-imidazolylphenyl)amine and cyanuric chloride, and then benzyl bromide was added to complete the quaternization of the total imidazolyl units. It featured a special exchangeable Cl^-/Br^--rich structure with high permanent porosity and wide pore size distribution, enabling it to rapidly and effectively remove environmentally toxic oxo-anions including Cr₂O₇^2-, MnO₄^-, and ReO₄^- and anionic organic dyes with different sizes including methyl blue, Congo red, and methyl orange from water. Notably, QUST-iPOP-1 showed ultra-high capacity values for radioactive TcO₄^- surrogate anions (MnO₄^- and ReO₄^-), Cr₂O₇^2-, methyl blue, and Congo red, and these were comparable to some reported compounds of exhaustive research. Furthermore, the relative removal rate was high even when other concurrent anions existed.

Foam fractionation for effective removal of Pseudomonas aeruginosa from water body: Strengthening foam drainage by artificially inducing foam evolution

Lack of microbial contamination is of great significance to drinking water safety and water reclamation. In this work, foam fractionation was employed to remove Pseudomonas aeruginosa (P. aeruginosa) from aqueous solution and dodecyl dimethyl betaine (BS12) was used as the collector. Since the attachment of strain cells on the bubble surface would impede the reflux of interstitial liquid in the plateau borders (PBs), a novel strategy in strengthening foam drainage was developed through artificially inducing foam evolution. Two gas distributors with different pore diameters had been mounted at the bottom of the column for regulating the radial distribution of bubble size in the foam phase. Experimental results indicated that gas diffuse and bubble coarsening could be significantly promoted by increasing the size difference among the adjacent bubbles. Bubble coalescence contributed to broadening the width of plateau borders, thereby avoiding the borders blockage by strain cells. During bubble coalescence, surfactant molecules would be partially shifted from the surface of small bubble towards that of large bubble due to the molecule density difference. The increase in surface excess of surfactant molecules on gas-liquid interface was conducive to improving foam stability. Under the suitable conditions of air flow rates of gas distributor with 0.125 mm of pore diameter 75 mL/min and gas distributor with 0.425 mm of pore diameter 125 mL/min, BS12 concentration 0.1 g/L, and P. aeruginosa concentration 2.0 × 10⁴ CFU/mL, the removal percentage and enrichment ratio of P. aeruginosa were 99.6% and 10.6, respectively. This work is expected to provide some new light for strengthening foam drainage in the presence of solid particles and to facilitate the industrialization of foam fractionation in water treatment.

An efficient separation for metal-ions from wastewater by ion precipitate flotation: Probing formation and growth evolution of metal-reagent flocs

Hazardous metal pollution became a severe environmental issue in China. An efficient precipitation-flotation process was developed to achieve fast removal for metal-ions from wastewater. Structure and strength of precipitate particles/flocs significantly influence the flotation removal of metal-ions. Formation and growth-evolution of precipitate flocs in precipitate flotation were studied by stage analysis of precipitate particles-formation, flocs-regulation and flotation separation. The results demonstrate that early formed precipitates MHA(humics-metal complexing particles) have small particle size, high fractal dimension, low strength and recovery factor. The addition of Fe³⁺ and CTAB(cetyl trimethyl ammonium bromide) reagents make the precipitate particles aggregated to flocs(MHA-Fe, MHA-Fe-CTAB) much more large, loose, coarse, and small-density. The final generated MHA-Fe-CTAB flocs are hard to be broken up, easy to be recovered and efficient to be separated by flotation process. The flotation removal of MHA-Fe-CTAB flocs is clearly higher than that of MHA or MHA-Fe. The flotation results of MHA-Fe-CTAB are as follows: flotation removal of 98.7 ± 0.40%-99.9 ± 0.10%, residual TOC of 0.96 ± 0.38-1.35 ± 0.41 mg/L and turbidity of 0.44 ± 0.09-0.63 ± 0.16 NTU. Introducing Fe³⁺ and CTAB reagents into flotation solution contributes to the growth-evolution of precipitate flocs, which could intensify the metal-ions removal via precipitate flotation process and result in more ideal purification indexes for metal-containing wastewater.

The key role of persistent free radicals on the surface of hydrochar and pyrocarbon in the removal of heavy metal-organic combined pollutants

We show that persistent free radicals (PFRs) on the surface of biochar can produce hydroxyl radicals (•OH) by catalyzing H₂O₂ to facilitate the removal of the combined pollutant BPA-Cr(VI). Microstructure characterization showed that the structures of pyrocarbon and hydrochar were significantly different when prepared at different temperatures. As the preparation temperature and preparation time for biochar increased, the concentration of PFRs first increased and then decreased. When biochar, PFRs, and H₂O₂ were present in the same solution, the single pollutants BPA and Cr(VI) as well as the combined pollutant BPA-Cr(VI) could be removed effectively, with removal rates greater than 90%. However, when PFRs, BPA, H₂O₂, and Cr(VI) were present in the same solution, Cr(VI) competed with H₂O₂ for electrons and promoted the removal of BPA. The results of this study could be applied to sludge recycling and be used to develop approaches to catalytically degrade combined pollutants.

A novel colloid composited with polyacrylate and nano ferrous sulfide and its efficiency and mechanism of removal of Cr(VI) from Water

Nano ferrous sulfide (n-FeS) colloids show an excellent performance in the application of remediation in situ soil and groundwater. However, due to the interfacial effect and high reactivity of the nano sized FeS, n-FeS easy to agglomerate, which reduces their remediation efficiency. In this study, a novel composite colloid was synthesized using polyacrylic acid salt (PAA) and n-FeS. The PAA-n-FeS colloid was used to remove Cr(VI) in water remediation, and its removal mechanism and efficiency were explored. The results showed that the hydrodynamic diameter of PAA-n-FeS ranged from 65.04-90.09 nm and the zeta potential was from -27 to -54 mV at pH varying from 4.5-9.0. PAA was coated on the surface of n-FeS, which improved the dispersibility and stability of n-FeS by increasing the steric hindrance and electrostatic repulsion between n-FeS particles. Moreover, the Cr(VI) maximum removal amount PAA-n-FeS was 432.79 mg/g, which was significantly higher than that of n-FeS (218.29 mg/g) and PAA (12.32 mg/g). The mechanism of PAA-n-FeS removal of Cr(VI) was mainly derived from its own reducibility. The reaction products were mainly Cr(OH)₃, Cr(III)-Fe(III), Cr₂O₃, and Cr₂S₃. This research not only finds a new stabilizer for preventing n-FeS agglomeration, but also provides a novel n-FeS composited colloid for promoting the practical application to Cr(VI) removal from water.

Foam Separation of Dyes Using Anionic, Cationic, and Amphoteric Surfactants

Foam separation can selectively remove a target substance from a solution via adsorption of the substance with the surfactant at the surface of the bubble. A cationic dye, methylene blue, and an anionic dye, Fast Green FCF, were prepared as substances to be removed via foam separation. Anionic (sodium dodecyl sulfate, SDS), cationic (dodecyltrimethylammonium chloride, DTAC), and amphoteric (3-(dodecyldimethylammonio)propane-1-sulfonate, SB-12) surfactants were used in the foam separation process. The effectiveness of the surfactants for removing the cationic methylene blue increased as follows: DTAC < SB-12 < SDS. On the other hand, the effectiveness of the surfactants for removing the anionic Fast Green FCF was in the opposite order. The dyes were effectively adsorbed by the foams via electrostatic interactions between the oppositely charged surfactant and the dye molecules. Since amphoteric surfactants have both anionic and cationic charges in a molecule, they could effectively remove both dyes in the foam separation process. Therefore, it was found that the amphoteric surfactant was highly versatile. Analysis of the kinetics of the removal rate showed that the aqueous solutions of monomers could remove the dyes more effectively than micellar solutions in foam separation.

Modified multi-walled carbon nanotubes assisted foam fractionation for effective removal of acid orange 7 from the dyestuff wastewater

In this study, multi-walled carbon nanotubes (MWCNTs) had been used to strengthen the removal of acid orange 7 (AO7) from the dyestuff wastewater by using foam fractionation. First, the surface modification of MWCNTs was performed by introducing hypochlorite groups (-OCl). The modified MWCNTs were characterized by using SEM, XRD, FTIR and Raman spectroscopy. Subsequently, the potential of modified MWCNTs as a novel collector for AO7 adsorption was examined. The adsorption conditions of modified MWCNTs towards AO7 were optimized by using response surface methodology (RSM) with a central composite design (CCD). The adsorption capacity of modified MWCNTs towards AO7 could reach 47.72 ± 0.79 mg·g^-1 under the optimum conditions. The kinetics and the equilibrium adsorption data were analyzed by using different kinetic and isotherm models. According to the regression results, adsorption kinetics data were well described by pseudo-second order model, whereas adsorption isotherm data were best represented by Langmuir isotherm model. Finally, foam fractionation was performed with a batch mode. Under the suitable conditions of loading liquid volume 300 mL, modified MWCNTs dosage 180 mg, cetyltrimethylammonium bromide (CTAB) concentration 50 mg·L^-1, AO7 concentration 30 mg·L^-1, pore diameter of gas distributor 0.125 mm and air flow rate 100 mL·min^-1, the removal percentage and enrichment ratio of AO7 were 91.23% and 6.17, respectively. The decolourization ratio of solution after foam fractionation was found to be 98.66%.

A self-gating proton-coupled electron transfer reduction of hexavalent chromium by core-shell SBA-Dithiocarbamate chitosan composite

We have proposed a novel strategy for the reduction plus adsorption process for hexavalent chromium elimination by thiol functional hybrid materials through a self-gating process. Namely, we exploit that coating dithiocarbamate chitosan at the surface of SBA-15 affords a core-shell composite that undergoes reversible shape transformations while thiol functional groups acted as proton-coupled electron donor for [Cr₂O₇]^2-. The reduction of [Cr₂O₇]^2- to Cr³⁺ was highly efficient and exceptionally rapid, occurred within 5 min with the reduction amount of 899.66 mg of [Cr₂O₇]^2- / 1 g of nanocomposite as a record high value. During the reduction of [Cr₂O₇]^2-, thiol functional groups (-SH) were oxidized into disulfide linkages (SS), and simultaneously chitosan matrix turned into shrunken structure because of the consuming of protons, preventing any release of Cr³⁺. Disulfides can also be reversely reduced to thiols by thiosulphates (S₂O₃^2-), which was attractive for regeneration and recyclability of the nanocomposite. Moreover, the [Cr₂O₇]^2- elimination through self-gating process was highly selective against a huge concentration of background electrolytes. This alternative strategy ensures the outstanding and stable performance in applied fields, and could be conducted in various pollution control techniques like permeable reactive barriers.

Bioleaching assisted foam fractionation for recovery of gold from the printed circuit boards of discarded cellphone

Present work was focused on recovering gold (Au) from the printed circuit boards (PCBs) of discarded cellphone by bioleaching assisted continuous foam fractionation. First, the cyanide-producing strains of Pseudomonas putida and Bacillus megaterium were co-cultured in order to supply a high cyanide concentration in the nutrient solution for mobilizing Au from waste PCBs (WPCBs). Bioleaching conditions were optimized by using response surface methodology. Under the suitable bioleaching conditions of pH of 10.0, pulp density of 5 g/L and leaching time of 34 h, the Au mobilization percentage was 83.59%. The leaching liquor with an Au concentration of 1.34 mg/L could be used as the feeding solution of continuous foam fractionation after removing solid particles and cell biomass. In order to strengthen foam drainage, a novel internal component of foam fractionation column was developed. Under the suitable operation conditions of CTAB concentration of 0.2 g/L, volumetric air flow rate of 100 mL/min and feed flow rate of 10 mL/min, the enrichment ratio and recovery percentage of Au were 43.62 and 87.46%, respectively. This study is expected to provide an effective strategy to recover Au from WPCBs, and to supplement the depleting natural resources.

Adsorption of Fluoroquinolone Antibiotics at the Gas-Liquid Interface Using Ionic Surfactants

The overuse of antibiotics in today's society has resulted in high concentrations of pharmaceutical contaminants in the natural environment. In this work, we investigated the surfactant-mediated adsorption of fluoroquinolone (FQ) antibiotics at the gas-liquid interface and their separation using a semi-batch foam fractionation process. FQs being a zwitterionic compound have an affinity to bind with both cationic and anionic surfactants. The adsorption of FQs to the gas-liquid interface was investigated using a cationic cetyltrimethylammonium bromide (CTAB) and anionic sodium dodecyl sulfate (SDS) surfactant. Ciprofloxacin (CF) was chosen as a model FQ antibiotic. The adsorption properties of the surfactant systems and CF were characterized using pendant-drop tensiometry and surface excess analysis. It was found that CF partitions to the gas-liquid interface more readily in the presence of SDS compared to CTAB. This was also corroborated in the foam fractionation experiments. At optimum operating conditions, CF showed a higher removal efficiency with SDS (96.3%) compared to CTAB (52%). In spite of strong molecular interactions between CTAB and CF, the preferential adsorption of DS^--CF complexes was far greater than CTA⁺-CF complexes. At optimized operating conditions, using SDS as the surfactant, other FQs such as norfloxacin, levofloxacin, and ofloxacin were recovered up to 97.9, 91.7, and 96.7%, respectively, with effluent concentration less than 100 nM. Overall, the work demonstrates foam fractionation as a novel and environment-friendly gas-liquid separation technique for the targeted removal of FQ antibiotics from waterbodies.

Separation of Protein-Binding Anthraquinones from Semen Cassiae Using Two-Stage Foam Fractionation

Anthraquinones are compounds of high medicinal value in many plants. Based on their good protein binding affinity, foam fractionation was attempted to separate them using proteins in the aqueous extract of Semen Cassiae as collectors. Firstly, the interaction between anthraquinones and Semen Cassiae proteins has been analyzed by the Stem–Volmer equation with physcion as a standard. The results show that physcion had good interaction with the proteins via hydrophobic forces. More importantly, the proteins effectively assisted the foam fractionation of several anthraquinones including aurantio-obtusifolin, aloe-emodin, rhein, emodin, chrysophanol, and physcion. On this basis, a two-stage foam fractionation technology was developed for process intensification using a foam fractionation with vertical sieve trays (VSTs). VSTs, initial feed concentration of total anthraquinones, temperature, volumetric air flow rate and pore diameter of gas distributor had significant effects on enrichment ratio and recovery yield of anthraquinones. Under suitable conditions, the enrichment ratio of total anthraquinones reached 47.0 ± 4.5 with a concentration of 939 ± 94 mg/L in the foamate while their total recovery percentage reached more than 47.7%. In addition, foam fractionation also increased the purity and hydroxyl radical scavenging activity of total anthraquinones. The results had significant implications for the separation of anthraquinones from Semen Cassiae.