Visualisation and characterisation of biopolymer clusters in a submerged membrane bioreactor

A laboratory wastewater treatment membrane bioreactor (MBR) with a submerged hollow-fibre membrane was used to investigate the major foulants in sludge mixtures. Confocal laser scanning microscopy (CLSM) with a triple fluorescent staining protocol, i.e., SYTO9 for microbial cells, ConA-TRITC lectin for polysaccharides and NanoOrange for proteins, was utilised to visualise the fouling materials. A pool of biopolymer clusters (BPCs) ranging from 2.5 to 60 μm in size was identified in the liquid phase of the MBR sludge and in the cake sludge on the membrane surface. According to the CLSM examination, BPC are free and independent organic solutes that are different from biomass flocs and extracellular polymeric substances (EPS) and much larger than soluble microbial products (SMP). Compared to EPS, BPC contain more polysaccharides and proteins and less humic substances. It is believed that BPC are an important foulant that interacts with biomass flocs to form the sludge fouling layer on the membrane. A filtration test observed with the CLSM shows that BPC are apparently formed by the adsorption and affinity clustering of SMP within the sludge deposited on the membrane surface. The cake sludge on the fouled membrane has a much higher BPC content (16.8 mg TOC/g SS) than the MBR bulk sludge (0.4 mg TOC/g SS). It is argued that BPC behave as a glue to facilitate the growth of an impermeable sludge cake on the membrane surface, thus resulting in serious MBR fouling. These CLSM findings provide the first direct evidence of the presence of BPC in MBR and illustrate their essential role in membrane fouling.     

Application of relaxation periods during electrodialysis of a casein solution: Impact on anion-exchange membrane fouling

Electrodialysis (ED) is a membrane process used on a large scale. However, one of the common problems is fouling of ion-exchange membranes stacked in the cell. The use of pulsed power, consisting in applying a constant current density during a fixed time of application (T_on) followed by a pause duration (T_off), was demonstrated recently as an effective fouling mitigation method for electrodialysis. Up until now, no work has investigated the potential of electrodialysis using pulsed electric field on protein fouling. The aim of the present work was to study the influence of pulsed electric field (PEF) with a low frequency square shaped periodic signal (T_on = 10 s–T_off = 10 s, T_on = 10 s–T_off = 40 s) in comparison with dc current during electrodialysis of a casein solution at different current densities (10, 20 and 30 mA/cm²) on membrane fouling. It appeared from these results that PEF, under certain conditions of pulse, would avoid fouling on anion-exchange membranes. For 10 s–40 s pulsed electric field conditions, no fouling was observed with any density, while for 10 s–10 s PEF conditions, fouling appeared only at current density over 10 mA/cm². dc current, whatever the current density conditions, led to a fouling on the diluate side of the AEM. Furthermore, when fouling occurred, magnitude layer thickness and dry weight increased with the applied current density. The nature of the fouling was identified as 97% protein. The protein fouling would be due to the dissociation of water molecules and/or heat increase at the anion-exchange membrane interface. The relaxation time of the pulse would limit both phenomena on the membrane.     

Crosslinked poly(ethylene oxide) fouling resistant coating materials for oil/water separation

Potential fouling reducing coating materials were synthesized via free-radical photopolymerization of aqueous solutions of poly(ethylene glycol) diacrylate (PEGDA). Crosslinked PEGDA (XLPEGDA) exhibited high water permeability and good fouling resistance to oil/water mixtures. Water permeability increased strongly with increasing the water content in the prepolymerization water mixture, going from 10 to 150 L μm/(m² h bar) as prepolymerization water content increased from 60 to 80 wt.%. However, molecular weight cutoff decreased as water content increased. These materials were applied to polysulfone (PSF) UF membranes to form coatings on the surface of the PSF membranes. Oil/water crossflow filtration experiments showed that the coated PSF membranes had water flux values 400% higher than that of an uncoated PSF membrane after 24 h of operation, and the coated membranes had higher organic rejection than the uncoated membranes.     

Influence of polypropylene membrane surface porosity on the performance of membrane distillation process

Two kinds of polypropylene capillary membranes were used in the membrane distillation (MD). These membranes exhibited a similar morphology, but one of them has an additional low porosity layer on the internal surface of capillaries. The changes of membrane performance during MD process of tap water were investigated. The presence of low porosity layer (thickness below 1 μm) caused that the air permeability was reduced from 1.365 to 0.863 dm³/m² s kPa, whereas the MD permeate flux was decreased only by 15%. A significantly larger decline of the flux was caused by CaCO₃ deposit formed during distillation of tap water. This deposit was removed every 30–70 h by rinsing the modules with a 2–5 wt.% HCl. Unfortunately, a repetition of this operation several times resulted in a gradual decline of the maximum permeate flux (distilled water as a feed). However, the module efficiency with the membranes covered by a surface layer of low porosity was found to decreases twice as slowly. The investigations revealed that a low surface porosity does not limit the possibility of surface wetting of polypropylene membranes, but hindered the scale formation inside the pores.     

Influence of inorganic scalants and natural organic matter on nanofiltration membrane fouling

The influence of inorganic scalants and NOM on nanofiltration (NF) membrane fouling was investigated by a crossflow bench-scale test cell. Mathematical fouling models were used to determine kinetics and fouling mechanisms of NF membrane. It was observed that, with natural organic matter (NOM) at a concentration of 10 mg L⁻¹, divalent cation, i.e. calcium (Ca²⁺), exhibited greater flux decline than monovalent cation, i.e. sodium (Na⁺), while solution flux curves dominated cake formation model, especially at high ionic strength. For inorganic scalants of polyanions, i.e. carbonate (CO₃²⁻), sulphate (SO₄²⁻), and phosphate (PO₄³⁻), solution flux curves were relatively fitted well with pore blocking model, possibly due to precipitated species formed and blocked on membrane surface and/or pores. For different divalent cations (i.e. calcium and magnesium (Mg²⁺)), calcium showed greater flux decline than magnesium, possibly due to higher concentration of precipitated calcium species than that of precipitated magnesium species based on the pC (−log concentration) and pH diagram.     

Single strand hollow fiber membrane (SSHFM): an on-line sample preparation for the flow based colorimetric determination of free iron in fruit juices

A single strand hollow fiber membrane (SSHFM) was developed for the on-line sample preparation for the flow based colorimetric determination of free iron levels in fruit juices. The SSHFM, as used, could separate Fe²⁺ from some spectrophotometric interfering agents in the fruit juice, such as pigments, solid suspensions and polysaccharides. The screening process was likely to have been primarily based on dialysis, wherein only ions or molecules that are smaller than the pores of the membrane can diffuse through while relatively larger molecules or particles could not. Two flow modes, a continuous and stopped flow, were studied. Factors that influenced the sensitivity (%dialysis) of the method, such as the flow rate, sample volume, flow direction and stopped flow time, were optimized. The stopped flow mode was found to be relatively more sensitive than the continuous flow mode and displayed a linear range of 1-30 mg L⁻¹ Fe²⁺, a limit of detection of 0.5 mg L⁻¹, and a % relative standard deviation of less than 2% (n = 8) for the analysis of 10 mg L⁻¹ Fe²⁺ spiked grape juice samples. A sample throughput of 24 samples h⁻¹, was attained without any further sample treatment.     

Moisture barrier,wetting and mechanical properties of shellac/agar or shellac/cassava starch bilayer bio-membrane for food applications

Edible bilayer membrane composed of agar (AG) or cassava starch (CAS) as a cohesive structural layer and ethanol-cast shellac layer as a moisture barrier are investigated for their potential use in food preservation as bio-packaging film, membrane or coating. Bilayer membranes containing non-plasticized shellac exhibit low water vapor permeability (WVP), from 0.89 to 1.03 × 10⁻¹¹ g m⁻¹ s⁻¹ Pa⁻¹. A high value of contact angle (≈92°) and a low liquid water adsorption rate (26 × 10⁻³ μL s⁻¹) indicate that these barrier layers have a quite hydrophobic surface. However, the rigid and brittle characteristics of shellac induce a lack of integrity for this layer. It tends to be cracked and scaled off. The incorporation of PEG 200 (plasticizer) into shellac improves the flexibility that prevents the defects in structure and reinforces the adhesion between the shellac and the cohesive-structural layer. The use of plasticizer weakly affects the WVP of bilayer membranes; however, the surface hydrophobicity as well as the liquid water adsorption rate is comparable to that of non-plasticized shellac layer. Furthermore, PEG increases the stretchability of bilayer membranes. Either being plasticized or not, shellac layer could improve significantly the functional properties of bilayer barriers and give a promising use as biopackaging.     

Structure-property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and meta-phenylene units III. Preparation and properties of semi-permeable membranes for water desalination by reverse osmosis separation performance

Flat sheet asymmetric reverse osmosis membranes were successfully prepared from N,N-dimethylacetamide (DMAc) solutions of a series of novel wholly aromatic polyamide-hydrazides that contained different amounts of para- and meta-phenylene rings. These polyamide-hydrazides were synthesized by a low temperature solution polycondensation reactions of either 4-amino-3-hydroxybenzhydrazide or 3-amino-4-hydroxybenzhydrazide with an equimolar amount of either terephthaloyl dichloride [TCl], isophthaloyl dichloride [ICl] or mixtures of various molar ratios of TCl and ICl in anhydrous DMAc as a solvent. All the polymers have the same structural formula except of the way of linking phenylene units inside the polymer chains. The content of para- to meta-phenylene moieties was varied within these polymers so that the changes in the latter were 10 mol% from polymer to polymer, starting from an overall content of 0-100 mol%. All the membranes were characterized for their salt rejection (%) and water permeability (cm³ cm⁻² day⁻¹) of 0.5 N aqueous sodium chloride feed solution at 3924 kPa operating pressure. The effects of polymers structural variations together with several processing parameters to achieve the best combination of high selectivity and permeability were studied. Effects of various processing parameters of the membranes on their transport properties were investigated by varying the temperature and period of the solvent evaporation of the cast membranes, coagulation temperature of the thermally treated membranes, annealing of the coagulated membranes, casting solution composition, membrane thickness and the operating pressure. During the thermal treatment step, the asymmetric structure of the membranes with a thin dense skin surface layer supported on a more porous layer was established. The former layer seems to be responsible for the separation performance. The results obtained showed that membrane performance was very much influenced by all of the examined processing variables and that membranes with considerably different properties could be obtained from the same polymer sample by using different processing parameters. Thus, the use of higher temperatures and longer exposure times in the protomembrane forming thermal treatment step would result in a membrane of lower solvent content and with a thicker skin layer and consequently led to higher salt rejection at lower water permeability. Most significantly, the membrane properties clearly depended on the polymer structure. Under identical processing condition, substitution para-phenylene rings for meta-phenylene ones within the polymer series resulted in an increase in salt rejection capability of the membranes. This may be attributed to an increase in their chain symmetry associated with increased molecular packing and rigidity through enhanced intermolecular hydrogen bonding. This produces a barrier with much smaller pores that would efficiently prevent the solute particles from penetration. Coagulation temperature controls the structure (porosity) of the membrane particularly its supported layer and consequently its water permeability. Moreover, annealing of the prepared membranes in deionized water at 100 °C was found essential for useful properties in the single-stage separation applications, which required optimum membrane selectivity. Upon annealing, the membrane shrinks resulting in reducing its pore size particularly in the skin layer and consequently improving the salt rejection. Addition of lithium chloride to the casting solution produced a membrane with increased porosity and improved water permeability. Salt rejection capability of the membranes is clearly affected by the applied pressure, reaching its maximum at nearly 4000 kPa. Furthermore, the water permeability is inversely proportional to the membrane thickness, while the salt rejection is not substantially influenced.     

Highly permeable mesoporous silica membranes synthesized by vapor infiltration of tetraethoxysilane into non-ionic alkyl poly(oxyethylene) surfactant films

Mesoporous silica membranes were prepared on porous alumina substrates by a vapor infiltration of tetraethoxysilane (TEOS) into a non-ionic poly(oxyethylene) (Brij56) surfactant film. Periodic mesostructured silica membranes were formed on both α- and γ-alumina substrates pre-treated with polystyrene. The polystyrene polymer plugged the pores of the alumina substrates and inhibited the deposition of silica in the alumina pores, resulting in the formation of a very thin silica membrane without a silica/alumina composite layer at the interface between mesoporous silica and the alumina substrates. The calcined mesoporous silica membrane showed very high nitrogen permeance (>10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹). The single gas permeation was governed by the Knudsen diffusion mechanism. The durability of the mesoporous silica membrane against moisture in air was improved by a silylation with trimethylethoxysiliane.     

Periodic air/water cleaning for control of biofouling in spiral wound membrane elements

The main problem during the operation of nanofiltration or reverse osmosis membrane plants is fouling of feed spacers in membrane elements due to biofouling and particulate fouling. In order to control biofouling and particulate fouling in membrane elements, both daily air/water cleaning (AWC) and daily copper sulphate dosing (CSD) were investigated and compared to a reference without daily cleaning. A pilot study was carried out for 110 days with three parallel spiral wound membrane elements; AWC, CSD and the reference which were fed by tap water enriched with a biodegradable compound (100 μg acetate-C/L). The CSD element, which combined daily copper sulphate dosing and sporadically air/water cleaning, performed best with an increase in pressure drop of 18% and a biomass concentration of 8000 pg ATP/cm² within 110 days. This was followed by the AWC element with a pressure increase of 37% and biomass concentration of 20,000 pg ATP/cm² within 110 days. The reference element showed a pressure increase of 120% within 21 days. The presented approach is considered very successful in controlling particulate fouling and biofouling, especially when air/water cleaning is combined with copper sulphate dosing.     

Rapid isolation of angiotensin peptides from plasma by electromembrane extraction

The present study has for the first time demonstrated the isolation of peptides from human plasma by electromembrane extraction (EME). Angiotensin 1, angiotensin 2, and angiotensin 3 migrated from 500 μL of diluted plasma, through a thin layer of 1-octanol and 8% di-(2-ethylhexyl) phosphate immobilized as a supported liquid membrane (SLM) in the pores of a porous hollow fiber, and into a 25 μL aqueous acceptor solution present inside the lumen of the fiber. The driving force for the extraction was a 15 V potential difference applied across the SLM. After only 10 min of EME, the peptides were isolated from diluted plasma (pH 3) with extraction recoveries between 25 and 43%. After optimization, the extraction system was evaluated using spiked plasma samples of angiotensin 2. The evaluation was performed by liquid chromatography electrospray mass spectrometry, showing linearity of angiotensin 2 in the range 2.5–125.0 ng/mL (r² = 0.989), and repeatability (RSD) between 5.6 and 11.6% (n = 6). The results demonstrate the possibility of isolating angiotensin peptides from plasma in only 10 min, using electromembrane extraction. The experimental findings are therefore promising with regard to future peptide extractions.     

Solvent-dependent permeability in asymmetric ceramic membranes with tortuous or non-tortuous mesopores

Solvent-dependent transport and the role of surface interactions were examined in commercial mesoporous ceramic membranes using permeability and thermoporometry measurements. The membranes chosen were titania (TiO₂) with tortuous interconnected pores (1, 5, and 50 kDa, corresponding to pore diameters of ca. 8.2, 18.3, and 33.2 nm, respectively) and alumina (Al₂O₃) with non-tortuous 20 nm cylindrical pores. A pre-water/solvent/post-water permeability cycle was employed to account for structural differences between membranes and to gauge the effect of residual solvent on water permeability at different temperatures. Our results suggest that in both types of membranes, restricted permeability of 1-butanol and cyclohexane was due to a combination of surface sorption and an increase in disjoining pressure due to solvation forces. Sorption and solvation forces were prevalent as their length scales were on the same order of magnitude as the pore radii. For 1-butanol, chemisorption changed the surfaces from hydrophilic to hydrophobic, and led to a significant decrease in post-water permeability. While Darcy's law could not describe 1-butanol and cyclohexane permeability, it did apply to water and 1,4-dioxane in the 20 nm alumina membranes. Thermoporometry, coupled with permeability, was further used to evaluate surface wetting within the mesopores.     

New type of nanofiltration membrane based on crosslinked hyperbranched polymers

Novel nanofiltration (NF) membrane was developed from hydroxyl-ended hyperbranched polyester (HPE) and trimesoyl chloride (TMC) by in situ interfacial polymerization process using ultrafiltration polysulfone membrane as porous support. Fourier transform infrared spectroscopy (FTIR-ATR), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and water contact angle (CA) measurements were employed to characterize the resulting membranes. The results indicated that the crosslinked hyperbranched polyester produced a uniform, ultra-thin active layer atop polysulfone (PSf) membrane support. FTIR-ATR spectra indicated that TMC reacted sufficiently with HPE. Water permeability and salts rejection of the prepared NF membrane were measured under low trans-membrane pressures. The resulting NF membranes exhibited significantly enhanced water permeability while maintaining high rejection of salts. The salts rejection increase was accompanied with the flux decrease when TMC dosage was increased. The flux and rejection of NF 1 for Na₂SO₄ (1 g/L) reached to 79.1 l/m² h and 85.4% under 0.3 MPa. The results encourage further exploration of NF membrane preparation using hyperbranched polymers (HBPs) as the selective ultra-thin layer.     

Bia-amperometric quantification of salbutamol in pharmaceutical products

This paper presents a simple, rapid and reproducible method of analysis of salbutamol in pharmaceutical products, utilizing batch injection analysis (BIA) associated with amperometric detection. A study of salbutamol oxidation demonstrated a strong dependence between electrode fouling and pH. All determinations were done utilizing a glassy carbon electrode in presence of 3.0 mol l⁻¹ NaOH. A large linear dynamic range from 8×10⁻⁷ to 2×10⁻⁴ mol l⁻¹ was obtained by using an injected volume of 100 μl with a detection limit of 2.5×10⁻⁷ mol l⁻¹. R.S.D. of 0.92% for 50 successive injections of 4×10⁻⁶ mol l⁻¹ of salbutamol and a sample throughput of 60 samples per hour were achieved. The method was applied for salbutamol quantification in syrups.     

Chlorine-induced permeability recovery for low-pressure membrane filtration of natural waters

Chlorine treatment is widely used by membrane filtration plants to recover the loss of membrane permeability encountered in low-pressure membrane (LPM) filtration of natural waters. However, there are few methodical studies in the literature addressing the efficacy of chlorine in cleaning membranes. Thus, the purpose of this study was to assess chlorine-induced permeability recovery (CIPR) of LPMs using the Ct concept (product of chlorine dose concentration and treatment time) commonly employed in the disinfection literature. The experimental work was conducted by evaluating the efficacy of CIPR for a membrane and water combination under variable Ct exposures and determining the presence of minimum effective Ct exposure and proper empirical models for the CIPR. The results showed that the efficacy of CIPR depended on both C and t. A minimum Ct exposure of approximately 2 × 10⁵ (min mg)/L was required for effective CIPR, and the relationship between the residual fouling and chlorine exposures was best fitted using a revised Chick–Watson model. These results may be explained by a conceptual model that considers CIPR as a sequential process of oxidation of organic foulants and diffusional detachment of the reaction products from membrane surfaces. Additional work is needed to validate the applicability of the model to other waters and membranes.     

Separation of casein micelles from whey proteins by high shear microfiltration of skim milk using rotating ceramic membranes and organic membranes in a rotating disk module

This paper investigates the microfiltration of skim milk in order to separate caseins micelles from two whey proteins, α-lactalbumin (α-La) and β-lactoglobulin (β-Lg), using a modified dynamic filtration pilot (MSD) consisting in 6 ceramic 9-cm diameter membrane disks of 0.2 μm pores, rotating around a shaft inside cylindrical housing. A comparison was made with another dynamic filtration module consisting in a disk rotating near a fixed PVDF 15.5 cm diameter membrane with 0.15 μm pores. Maximum permeate fluxes were 120 L h⁻¹ m⁻² with the MSD module at 1930 rpm and at 40 °C, and 210 L h⁻¹ m⁻² at 2500 rpm and 45 °C, with the rotating disk module. Casein rejection was around 99% at high speed for both membranes. α-La transmission decreased with increasing transmembrane pressure (TMP) from 75% to 60% for ceramic membranes and from 25% to 10% for the PVDF one. β-Lg transmissions were lower, ranging from 23% to 15% for ceramic membranes and from 20% to 5% for the PVDF one. In a concentration test with the PVDF membrane at 2000 rpm, the flux decayed from 200 L h⁻¹ m⁻² at initial concentration to 80 L h⁻¹ m⁻² at VRR = 3.2 and 22.1% of the initial α-La mass was recovered in the permeate, against 8.1% for β-Lg. Permeate fluxes in the mass transfer limited regime (J_lim) of the MSD and rotating disk module operated at various speeds were well correlated by the equation J_lim = 17.13 V_av where V_av denoted the disk azimuthal velocity averaged over the membrane area. Measurements of J_lim, taken from Ref. [G. Samuelsson, P. Dejlmek, G. Tragardh, M. Paulsson, Minimizing whey protein retention in crossflow microfiltration of skim milk. Int. Dairy J. 7 (1997) 237–242] during MF of skim milk using tubular ceramic membranes at velocities from 1.5 to 8 m s⁻¹ with permeate co-current recirculation were found to obey the same correlation.     

A new concept of hollow fiber liquid–liquid–liquid microextraction compatible with gas chromatography based on two immiscible organic solvents

A new concept of liquid–liquid–liquid microextraction (LLLME) was introduced based on applying two immiscible organic solvents in lumen and wall pores of hollow fiber (HF). With this methodology, analytes of interest can be extracted from aqueous sample, into a thin layer of organic solvent (dodecane) sustained in the pores of a porous hollow fiber, and further into a μL volume of organic acceptor (acetonitrile or methanol) located inside the lumen of the hollow fiber. Some chlorophenols (CPs) were selected as model compounds for developing and evaluating of the method performance. The analysis was performed by gas chromatography–electron capture detection (GC–ECD) without derivatization. The factors affecting the HF-LLLME of target compounds were investigated and the optimal extraction conditions were established. Under the optimum conditions, preconcentration factors in a range of 208–895 were obtained. The performance of the proposed method was studied in terms of linear dynamic ranges (LDRs from 0.02 to 100 ng mL⁻¹), linearity (R² ≥ 0.995), precision (RSD % ≤ 8.1) and limits of detection (LODs in the range of 0.006–0.2 ng mL⁻¹). In addition to preconcentration, HF-LLLME also served as a technique for sample clean-up.     

Systematic investigation on new SrCo1−yNbyO3−δ ceramic membranes with high oxygen semi-permeability

SrCo_1−yNb_yO_3−δ (y = 0.025–0.4) were synthesized for oxygen separation application. The crystal structure, phase stability, oxygen nonstoichiometry, electrical conductivity, and oxygen permeability of the oxides were systematically investigated. Cubic perovskite, with enhanced phase stability at higher Nb concentration, was obtained at y = 0.025–0.2. However, the further increase in niobium concentration led to the formation of impurity phase. The niobium doping concentration also had a significant effect on electrical conductivity and oxygen permeability of the membranes. SrCo_0.9Nb_0.1O_3−δ exhibited the highest electrical conductivity and oxygen permeability among the others. It reached a permeation flux of ∼2.80 × 10⁻⁶ mol cm⁻² s⁻¹ at 900 °C for a 1.0-mm membrane under an air/helium oxygen gradient. The further investigation demonstrated the oxygen permeation process was mainly rate-limited by the oxygen bulk diffusion process.     

Synthesis of novel crosslinked sulfonated poly(ether sulfone)s using bisazide and their properties for fuel cell application

Novel crosslinked sulfonated poly(ether sulfone)s (PESs) were prepared by thermal irradiation of the allyl-terminated telechelic sulfone polymers using a bisazide. The sulfonated polymers in different comonomer compositions were fully characterized by ¹H NMR, and the crosslinked structure was also verified by FT-IR spectroscopic analyses. Having both the uniform distribution of the hydrophilic conductive sites and controlled hydrophobic nature by minimized crosslinking over the rigid rod poly(ether sulfone) backbone, the crosslinked polymer membrane (PES-60) offered excellent proton conductivity of 0.79 S cm⁻¹ at 100 °C together with hydrolytic and oxidative stability. In addition, only 17% of methanol permeability of the Nafion^® was observed for the crosslinked PES-60.     

Determination of Pb in natural waters with 2-mercaptobenzimidazole-5-sulfonate (MBIS)chemically modified gold electrode

The preparation, characterization and analytical applications of gold electrodes modified with ω-mercapto alkyl/aryl sulfonates are described. The devices resulted effective for the determination of free and labile Pb(II) in water samples by anodic stripping voltammetry (E_dep = −600 mV) and in particular 2-mercaptobenzimidazole-5-sulfonate (MBIS) offered the best performances with detection limit of 0.4 μg L⁻¹.Quantitative stripping of Pb from the electrode surface during the anodic scan, is obtained using sodium citrate buffer (pH = 9.0) as supporting electrolyte. Natural waters were analysed by standard addition method with good recoveries (mean percentage = 97%); no fouling effects due to humic acids or other organic constituents were observed in the reported conditions.