Favorable pH Value Research Articles

In situ encapsulation of capsaicinoids in MIL-88A as a food-grade nanopreservative for meat safety

The biocompatible MIL-88A metal-organic framework (MOF), synthesized from food-grade fumaric acid and ferric chloride, was introduced for the efficient one-step in situ encapsulation of capsaicinoids as a nanopreservative. The resulting MIL-88A@Caps nanoparticles can load 61.43 mg/g of capsaicinoids, surpassing conventional MOF-based encapsulation. The potent MIL-88A@Caps nanoformulations synergize the intrinsic antimicrobial properties of MIL-88A and capsaicinoids. At the same concentration (0.5 mg/mL), MIL-88A@Caps was highly effective against S. aureus and Salmonella, with inhibition rates of 94.90 ± 0.58% and 94.30 ± 1.24%, respectively, compared to MIL-88A (62.28 ± 5.04% and 70.46 ± 1.96%) and capsaicinoids (63.68 ± 1.25% and 49.53 ± 1.22%), respectively. Model precooked-chicken preservation experiments revealed that MIL-88A@Caps significantly delayed spoilage parameters compared to untreated samples, with more favorable viable counts (8.08 lgCFU/g), pH value (6.60 ± 0.02), TVB-N value (8.59 ± 0.21 mg/100 g), and color changes on day 9. Our findings yield a green nanopreservative for meat safety.

The Use of Beech Bark (Latin: Fagus sylvatica) and Birch Bark (Latin: Betula pendula Roth) for the Removal of Cationic Dyes from Aqueous Solutions

The aim of this work was to determine the sorption capacity of the cationic dyes Basic Red 46 (BR46) and Basic Violet 10 (BV10) on the prepared sorbents: beech bark (BBe) and birch bark (BBi). Two fractions of bark were used in the research: fine (2–3 mm) and coarse (4–5 mm). The carried out tests made it possible to determine the influence of the pH value on the sorption efficiency, the sorption equilibrium time and the maximum sorption capacity of the two tested sorbents. The Langmuir model and the Freundlich model were used to describe the obtained experimental data. Beech and birch barks are effective sorbents for cationic dyes; however, the efficiency of dye sorption on both bark sorbents depends on the type of cationic dye. According to the obtained data, beech and birch bark sorbents showed higher sorption efficiency for Basic Red 46 than for Basic Violet 10. The pH correction was a necessary condition for sorption, and the sorption pH value for the cationic dyes Basic Red 46 and Basic Violet 10 was be determined individually for each dye. The most favourable pH value for the sorption of the BR46 dye on the beach and birch bark sorbents was pH = 6, while for the dye BV10, it was pH = 3. The sorption equilibrium time for Basic Red 46 was 300 min and for Basic Violet 10–240 min. The fine fraction of beech bark had the highest sorption capacity for both BR46 (128.45 mg/g dry matter) and BV10 (18.07 mg/g dry matter).

Development of Effervescent Cleansing Tablets Containing Asiatic-Acid-Loaded Solid Lipid Microparticles

The objective of this study was to develop effervescent cleansing tablets that can be dissolved and turned into liquid soap, which can be used for bathing or soaking the body. The asiatic-acid-loaded solid lipid microparticles (AASLMs) were prepared via the hot emulsification method followed by cold re-solidification and then freeze-dried to obtained dry powder. The physicochemical properties such as morphology and % entrapment efficiency (%EE) were evaluated. The results revealed that AASLMs have an irregular shape, and the %EE for the resulting AASLMs was 92.04 ± 3.43%. The tablets were manufactured via the direct compression technique. The compatibility test was conducted to ensure that the excipients are compatible with the active ingredient. The angle of repose, Carr’s index, and Hausner’s ratio were studied to evaluate the flowability of the powder blend before compression. The weight of each tablet was set to 1000 mg, and physicochemical characteristics, in vitro dissolution, ex vivo cleansing efficacy, and stability were evaluated. The results showed that the active ingredient was compatible with other excipients, as the results obtained from FTIR spectra indicated the absence of potential chemical interaction between the active ingredient and excipients used in this study. Additionally, all formulations had good flow properties. The effervescence times of selected formulations, F2 and F3, were <5 min, with favorable pH and hardness values. The friability values of all formulations exceeded 1% because the excipients used in effervescent tablets are very fragile. The release of asiatic acid (AA) from the tablets was dependent on the concentration of SLS. In an ex vivo test, it was discovered that the developed products F2 and F3 showed much more effective cleansing efficacy than water. Nevertheless, brown spots appeared in the tablets and the AA content was significantly decreased in both tested formulations after 3 months’ storage at 40 ± 2 °C/75% RH ± 5% RH. The stability study revealed that the developed products were not stable at high temperature and humidity. Therefore, it is recommended that the developed effervescent tablets are not stored at a high temperature.

Using Agricultural Mixed Waste as a Sustainable Technique for Removing Stable Isotopes and Radioisotopes from the Aquatic Environment

In the current study, dried ground mixed waste of olive waste and water hyacinth was subjected to the treatment of wastewater contaminated with stable or radioactive cobalt and cesium. Contact time, temperature, amount of adsorbent, metal ion concentration and pH-value were evaluated as the most potent factors that affect the adsorption process. Concentrations of Co and Cs ions before and after treatment are measured using ICP-OES technique. Moreover, kinetic and equilibrium isotherm parameters were investigated by explaining the equilibrium data by induction of two isotherms, “Langmuir” and “Freundlich”. Experimental results indicated that more than 85% of 60Co and 134Cs were efficiently removed from spiked wastewater after one hour of contact time by using 0.2 g of dried mixed waste. The optimal time to remove Cs+ and Co2+ was 50 and 90 min, respectively. The kinetic study showed that the adsorption of Cs+ and Co2+ were better suited to the second order. The most favorable pH value was at a range of 6–7 for Cs+ and 5–6 for Co2+. R2 values were higher at the Freundlich isotherm, indicating that the adsorption process was taking place according to Freundlich isotherm at all temperatures for Co2+ and Cs+ ions.

Coconut flower sheath derived activated charcoal as efficient and cost effective adsorbent for crystal violet dye removal

In this study, the ability of acid-activated charcoal derived from coconut flower sheath is investigated as an efficient and low-cost adsorbent for hazardous cationic dye removal. The crystal violet dye used in the current study is an extensively used industrial dye and its adsorption studies conducted by using activated charcoal exhibit efficient dye removability. The results obtained from dye adsorption studies illustrate a favorable pH value ∼ 9 with a 1.5 g/250 ml adsorbent dosage. The adsorption studies also demonstrate contact time around 40 min for maximum dye removal with an efficiency of 94%. The hydrochloric acid-activated charcoal adsorbed 35 mg/L of dye whereas the phosphoric acid activated charcoal adsorbed 55.55 mg/L respectively. The adsorption mechanism observed for both activated charcoals fits well with Langmuir isotherm and kinetic data followed pseudo second-order model.

Chitosan/β-glycerophosphate in situ forming thermo-sensitive hydrogel for improved ocular delivery of moxifloxacin hydrochloride

The aim of the current work is to develop a thermo-sensitive hydrogel system of moxifloxacin hydrochloride (MOX) for improved ocular delivery. Fifteen formulations were prepared at different concentrations of β-glycerophosphate disodium salt (β-GP) 12–20% (w/v) and chitosan (CS) 1.7–1.9% (w/v). The optimized MOX loaded thermo-sensitive hydrogel system (F8), consisting of CS (1.8%, w/v) and β-GP (16%, w/v), showed optimum gelation temperature (35 °C) and gelation time (2 min), thus was selected for further investigations. It showed a significant decrease (p < 0.05) in the zeta potential value compared to CS solution with a favorable pH value (7.1) and confirmed thermoreversible behavior. MOX loaded F8 displayed a porous structure under scanning electron microscopy. Rheological investigation of MOX loaded F8 revealed the presence of a strong hydrogel network with high elasticity along with a small loss factor of 0.08 indicating a great ease of gel formation. The release of MOX from F8 was found to be governed by a combined mechanism of diffusion and relaxation. Biological assessment of two concentrations of MOX loaded F8 (0.25 and 0.5%) was conducted using healthy and infected male albino New Zealand rabbits, where an improved and prolonged antibacterial activity against Staphylococcus aureus compared to plain MOX (0.5%), marketed MOX eye drops (0.5%), was shown. Moreover, histopathological examination of ocular tissues confirmed the antibacterial efficacy of the optimized formulation eight days post topical therapy. Consequently, the developed CS/β-GP thermo-sensitive hydrogel system (F8) reveals a promising potential for enhancing the ocular delivery of MOX for treatment of bacterial infections.

The Influence of Proteolytic Malt Modification on the Aging Potential of Final Wort.

The dynamic changes in beer flavor are determined by its aging potential, which comprises of present free and bound-state aldehydes and their precursors. Rising flavor-active aging compounds cause sensory deterioration (flavor instability). These compounds are mainly formed upstream in the brewing process through the Maillard reaction, the Strecker degradation, or lipid oxidation. Wort boiling is an especially critical production step for important reactions due to its high temperature and favorable pH value. Amino acid concentration, as an important aging-relevant precursor, is variable at the beginning of wort boiling, mainly caused by the malt modification level, and can further influence the aging potential aging formation during wort boiling. This study investigated the effect of the proteolytic malt modification level on the formation of precursors (amino acids and dicarbonyls) and free and bound-state aldehydes during wort boiling. Six worts (malt of two malting barley varieties at three proteolytic malt modification levels) were produced. Regarding precursors, especially Strecker, relevant amino acids and dicarbonyls increased significantly with an enhanced malt modification level. Concentrations of free and bound aldehydes were highest at the beginning of boiling and decreased toward the end. A dependency of malt modification level and the degree of free and bound aldehydes was observed for 2-methylpropanal, 2-methylbutanal, and 3-methylbutanal. Generally, a higher proteolytic malt modification level tended to increase free and bound aldehyde content at the end of wort boiling. Conclusively, the aging potential formation during boiling was increased by an intensified malt modification level.

Towards Optimal pH of the Skin and Topical Formulations: From the Current State of the Art to Tailored Products

Acidic pH of the skin surface has been recognized as a regulating factor for the maintenance of the stratum corneum homeostasis and barrier permeability. The most important functions of acidic pH seem to be related to the keratinocyte differentiation process, the formation and function of epidermal lipids and the corneocyte lipid envelope, the maintenance of the skin microbiome and, consequently, skin disturbances and diseases. As acknowledged extrinsic factors that affect skin pH, topically applied products could contribute to skin health maintenance via skin pH value control. The obtained knowledge on skins’ pH could be used in the formulation of more effective topical products, which would add to the development of the so-called products ‘for skin health maintenance’. There is a high level of agreement that topical products should be acidified and possess pH in the range of 4 to 6. However, formulators, dermatologists and consumers would benefit from some more precise guidance concerning favorable products pH values and the selection of cosmetic ingredients which could be responsible for acidification, together with a more extensive understanding of the mechanisms underlaying the process of skin acidification by topical products.

Synthesis, characterization and analytical applications of Ni(II) ion-imprinted polymer prepared by N-(2-hydroxyphenyl)acrylamide

In the present study, a novel Ni(II) ion-imprinted polymer (Ni(II)-IIP) was successfully synthesized by copolymerization of N-(2-hydroxyphenyl)acrylamide as a functional monomer, 2,2′-Azobis(2-methylpropionitrile) (AIBN) as an initiator and ethylene glycol dimethacrylate (EGDMA) as a cross-linker in the attendance of nickel nitrate as a template ion. The nickel ions in the Ni(II)-IIP were eliminated by ethylenediaminetetraacetic acid (EDTA) leaching. The functional monomer, complex and the obtained polymer particles were characterized by 1H NMR, FT-IR, CHNS, FESEM and the concentration of Ni(II) ions in the solution were determined by flame atomic absorption spectrophotometer (FAAS). The effects of pH, flow rate, temperature, sample concentration, sorbent particle size, desorption eluent conditions, selectivity and reusability on the extraction of Ni(II) ions were surveyed. The results of the study revealed that most favorable pH value for quantitative enrichment was 7 and maximum adsorption capacities of Ni(II) ions on the IIP was 38 mg/g. In addition, the relative standard deviation (RSD) for six separately experiments with a nickel concentration of 20.0 μg/L was 2.72% and limit of detection was found to be 0.7 μg/L.

Evaluation of a combination of phosphorylated fibers and zeolite as a potential substitute to synthetic wetting agents in peat moss products

Water availability and pH are important factors to consider to determine the suitability of a material for use as a growing medium. Unfortunately, most horticultural substrates are characterized by their water repellency. This is the case with peat moss which is hydrophobic and acidic. Synthetic surfactants are required to improve its wettability. In this study, a combination of phosphorylated wood pulp fibers (FLP) and zeolite is proposed as a substitute to surfactants to increase the wettability of peat moss in the presence of lime, an additive generally used as fertilizer or pH regulator. Results show that lime reduces the water retention capacity of FLP. However, the addition of 15% zeolite to the peat moss/FLP system increases the pH and water retention of the substrate. The negative effect of the presence of 1 wt. % lime on the water retention of the peat moss/FLP mixture was corrected by zeolite addition. Optimal conditions were obtained at 10% zeolite for the two types of lime tested with favorable pH and water retention capacity values. Zeolite was shown to have a higher affinity than FLP for calcium ions preventing the detrimental interaction between FLP and calcium ions.

Enhancing methanogenesis in fed-batch anaerobic digestion of high-strength sulfate-rich wastewater using zero valent scrap iron

The aim of this study was to evaluate the addition of zero valent iron (ZVI) in scrap form to an anaerobic digestion reactor in order to enhance methane production from high-strength, sulfate-rich wastewater. The wastewater contained 6,000 mg/L COD and 3,000 mg/L sulfate. From an initial set of batch tests, the addition of either ZVI powder or ZVI scrap to the medium resulted in comparable methane yields of around 0.13 L CH4/g COD added. In a fed-batch operation treating the sulfate-rich wastewater for 60 days, the reactor with added ZVI scrap was found to have higher media pH (6.3-8.1), increased sulfate reduction efficiency (65-85%) and higher concentrations of dissociated sulfide (106-1,020 mg S/L) than a fed-batch operation without added ZVI scrap. The reactor with added ZVI had a maximum methane yield of 0.25 L CH4/g COD added⋅day and a maximum methane composition in the biogas of 53%. In contrast, the control reactor without added ZVI gave a methane yield of 0.07 L CH4/g COD added⋅day and a methane composition in the biogas of 27%. The control reactor also had higher concentrations of undissociated sulfide (153-612 mg S/L) and relatively high volatile fatty acids (VFA) to alkalinity ratios (0.4-0.7 g CH3COOH/g CaCO3). The experimental results suggested that ZVI provided favorable pH values for methanogenesis, reduced the toxicity of sulfides and thus enhanced the competitiveness of methanogens with sulfate-reducing bacteria (SRB). Moreover, the larger negative oxygen reduction potential (ORP) values in the reactor with added ZVI scrap indicated that the ZVI might enhance electron transfer activities of microbes in the reactor.

Post-Coagulation Sludge as an Adsorbent of Dyes from Aqueous Solutions

Abstract The main source of usable water supply in Poland is surface water. The most frequently used methods of surface water treatment are coagulation and filtration processes. During these processes post-coagulation sludge is generated, which is the main waste produced in water treatment plants. This sludge is characterized by an amorphous structure with a strongly developed specific surface, similar to sewage sludge. Dried and crushed post-coagulation sludge was used in the studies as an adsorbent of two acid dyes: Acid Red 18 (AR 18) and Acid Green 16 (AG 16). The most favourable pH of process and contact time as well as sorption isotherm were established. For both dyes the most favourable pH value was equal 2. The tests showed that in the case of AR 18 (180 min) a longer contact time was necessary to reach the equilibrium than for AG 16 (120 min). Moreover, based on the different isotherm two-parameter models, the sorption capacity, type of sorption and energy of sorption were calculated based on linear and non-linear regression. The tests showed that in the case of dye AG 16 monolayer sorption occurred - the analysis showed better fitting the results to Langmuir model. Moreover, for both dyes, physical adsorption took place - it was indicated by the values of parameter 1/n (computed from the Freundlich model).

Preparation and Removal Properties of Cimetidine from Aqueous Solution by Waste Bricks Incorporated with Different Iron Oxides

In this study, to investigate the elimination of micropollutant from aqueous solutions by construction waste, waste red brick (WRB) with and without incorporated of iron species (goethite and hematite) were prepared by a simple method. The prepared materials were systematically characterized and batch experiments were conducted to study the elimination of typical micropollutant cimetidine from the water environment. Results showed that both two iron species could be successfully incorporated onto WRB and the main structure of WRB were maintained. The adsorption process of cimetidine onto bricks was fast due to the microstructure and useful adsorption sites on the surface of bricks. The Langmuir model fitted the experimental data better and the qmax of cimetidine increased about 24.4% and 39.6% for the incorporation of hematite and goethite, respectively. The pH values influenced the adsorption behavior greatly and the favorable pH value was around 6.0. The charge screening effect and competition adsorption may influence the adsorption behavior together. Due to the interaction between cimetidine and bricks, the outer-sphere complexation may be formed in the adsorption process. This study shows new methods for the elimination of the micropollutant from the water environment and offers useful guidelines for the reuse of construction waste.

Biochar efficiency in copper removal from Haplic soils

The main aim of the study was to examine the biochar efficiency in copper (Cu) ion removal from Haplic (Luvisol, Podzol) soils. In other words, the most effective biochar dose and the most favorable environment pH value were proposed. The copper removal was carried out based on the adsorption process. The results of kinetics measurements were modeled using Richie equation, whereas the adsorption isotherms using Langmuir–Freundlich and Dubinin–Radushkevich equations. The copper removal is a strongly pH-dependent process which is associated with variability of chemical Cu forms and various reactivities of organic molecules. The Cu adsorption amount was higher on silty Haplic Luvisol than on sandy Haplic Podzol mainly due to higher content of organic carbon, functional groups, clay and silt fractions as well as larger surface area. Moreover, the biochar addition increased linearly the soil surface charge and contributed to higher copper adsorption in the whole range of investigated ion concentrations. The higher the biochar dose added, the more Cu ions were adsorbed.

Influence of solvent additives on the aqueous extraction of tannins from pine bark: potential extracts for leather tanning

AbstractBACKGROUNDPine bark, which represents approximately 10 to 20% in weight of the tree trunk, is an abundant and low cost agro‐industrial waste. In order to obtain condensed tannin‐rich pine bark extracts (for future application in leather tanning/retanning processes), conventional aqueous extractions were performed, varying the type and amount of the solvent additive: sodium hydroxide (0.5, 1.0 and 1.5%, w/v), formic acid (0.5, 1.0 and 1.5%, v/v) and ethanol (5, 10 and 15%, v/v). Extractions were performed with and without the addition of sodium sulfite (1.0%, w/v).RESULTSThe most favorable solvent additive in terms of total phenolics and condensed tannins was ethanol: extracts achieved 34.8% of gallic acid equivalents and 62.8% of catechin monohydrate equivalents. Moreover, the favorable pH values (∼3.5) of the resultant aqueous solutions and their relatively low viscosities are suitable for subsequent tanning applications.CONCLUSIONSThe results of this study suggest that the high added‐value aqueous extracts of Pinus pinaster bark, which were achieved with small amounts of ethanol, have appropriate characteristics for leather tanning, and therefore they may be used as eco‐friendly tanning agents to partially replace chromium salts, thus reducing some of the environmental impacts typically associated with the leather industry. © 2017 Society of Chemical Industry

Functionally reduced graphene oxide supported iron oxides composites as an adsorbent for the immobilization of uranium ions from aqueous solutions

Comprehension the extent and rate of U(VI) adsorption on some mineral surfaces are significant to predict their migration and transformation properties in the environmental systems. Herein, iron oxides particles supported reduced graphene oxide composites (MRGO) were successfully synthesized by in-situ chemical precipitation approach. The MRGO composites which illustrated high saturation magnetization could be likely separated using an external magnet. The experimental data illustrated that U(VI) ions could be interacted with MRGO composites through different interaction mechanisms depending on different environmental conditions. The adsorption kinetic results can be well simulated using the pseudo-second-order pattern. The pH-dependent adsorption behavior illustrated an favorable pH value of 7.5 for removal of U(VI) ions using MRGO composites. Furthermore, the adsorption regeneration of MRGO composites in simulated wastewater disposal systems was tested. Based on above mentioned, it was clear that MRGO composites could be potentially used as adsorbent for removal of actual U(VI)-bearing wastewaters.

The effect of carbohydrate additive and inoculation on quality of red clover silage

In this experiment, wilted masses of red clover of cultivar K-17 from the first cut was ensiled in three treatments: a) no additives, b) with the addition of corn (6% of biomass) and c) with the addition of inoculant BioStabil Plus. The experiment design was according to the method of a completely random plan (single factorial trial) in triplicates. Based on the results it can be concluded that the wilted biomass of red clover can be successfully ensiled without additives. However, the inoculation of red clover biomass achieves the most favourable pH value (4.20), the lowest level of degradation of the protein expressed in the amount of NH3-N (107.7 gkg-1 N), the largest production of lactic acid (91.3 gkg-1 DM) and acetic acid (42.6 gkg-1 DM), in the absence of butyric acid. Adding maize meal in the amount of 6% contributed to somewhat more favourable fermentation and increase of the energy value of silage. When using the DLG and Weissbach methods for assessing the quality of silage, all silages were classified into the first class. Contrary to this, according to the Zelter method, control and inoculated silages were evaluated as class III, because of the large amounts of acetic acid. In practices inoculants based on homo-and hetero-fermentative bacteria of lactic acid fermentation are recommended for use, because the increased production of acetic acid contributes positively to te aerobic stability of silage.

Effects of operating parameters on efficiency of lead removal by complexation-microfiltration process

Majority of lead content found in the environment is the result of human activities. Heavy metals can be hazardous because they tend to bioaccumulate. Complexation-microfiltration process for the removal of Pb(II) ions was studied. The aim of microfiltration of the model wastewater containing heavy metal ions was finding an optimum ratio between the concentrations of the complexing agent and metal, and determining the most favorable pH value. The microfiltration experiments were carried out in a stirred dead-end cell. Diethylaminoethyl cellulose (DEAE 23) was selected as the complexing agent. Versapor membranes were used to separate formed polymer-metal complex. The concentration of heavy metal ions after microfiltration in aqueous solution was determined using the atomic absorption spectroscopy (AAS). Effects on the amount of complexing agent, concentration of metal ion, pH value and operating pressure on the flux, J, and rejection coefficient, R, were investigated. Experimental results indicate that the pH of the solution has considerable influence on the rejection coefficient. An increase in pH and the amount of complexing agents enabled us to obtain very high retention coefficient (99%).

Biosorption of aluminum ions onto Rhodococcus opacus from wastewaters

This fundamental work deals with the biosorption removal of Al (III) using a Rhodococcus opacus strain. Several variables that have an effect on the capacity of aluminum biosorption from water streams by R. opacus were studied, particularly the effects of solution pH, biosorbent concentration, metal concentration, contact time, ionic strength and temperature of biosorption. The most favorable biosorption pH value of Al (III) was determinate as around 5.0 and the maximum sorption capacity was found to be 41.59 mg g −1 at pH 5.0 (initial concentration of 50 mg L −1) and temperature 25 °C. The experimental data obtained have been analyzed using four two-parameter models (Langmuir, Freundlich, Temkin and Dubinin-Radushkevich) and two three-parameter models (Redlich–Peterson and Sips). In order to determine the best fit isotherm, three error analysis methods were used to evaluate the data: correlation coefficient, chi-square and residual root mean square error. Dynamics of sorption process were studied and the values of rate constant of biosorption were calculated. The uptake capacity is a function of pH solution, ionic strength and biosorbent concentration.

Detoxification of mercury by immobilized mercuric reductase

Mercuric reductase which originated from a recombinant Escherichia coli PWS1 was purified and immobilized on a chemically modified diatomaceous earth support. The mercury reduction kinetics, pH dependence, storage stability, and reusability of the immobilized enzyme were investigated. Four dyes were examined for their electron transfer efficiency with the soluble and bound mercuric reductase. Continuous mercury detoxification by the immobilized mercuric reductase was also performed in fixed-bed processes. The effects of bed-length, mercury loading rate, and electron donor on the performance of the fixed beds were assessed. Immobilized mercuric reductase exhibited substrate-inhibition-type kinetics with a maximal activity (1.2 nmol Hg mg−1 protein s−1) occurring at an initial Hg2+ concentration of 50 µmol dm−3. The optimal pH was 7.0 for the soluble and immobilized mercuric reductase, but the immobilized enzyme maintained higher relative activity for less favorable pH values. Immobilization of the enzyme appeared to significantly enhance its storage stability and reusability. Of four artificial electron donors tested, azure A (5 mmol dm−3) demonstrated the highest relative activity (78%) for soluble mercuric reductase. For the immobilized enzyme, neutral red (5 mmol dm−3) gave a relative activity of nearly 82%. With a fixed-bed, the mercury-reducing efficiency of using neutral red was only 30–40% of that obtained using NADPH. Fixed-bed operations also showed that increased bed length facilitated mercury reduction rate, and the optimal performance of the beds was achieved at a flow rate of approximately 100–200 cm3 h−1. © 1999 Society of Chemical Industry