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Content archived on 2024-04-19

The removal and recovery of cadmium by biosorption, flotation and electrolysis


A project has been set up to identify and evaluate strains of the genus streptomyces with excellent cadmium binding characteristics. Commercially used streptomycetes, strains in the Newcastle collection, and newly isolated strains from metal rich soils were examined. For commercial reasons biomass waste from a pharmaceutical process would have to be provided in a form from which live microorganisms could not be isolated. For this reason, work has been carried out with streptomyces biomass killed by autoclaving in the liquid culture medium in which it was grown. 2 antibiotic producing commercial strains were chosen for preliminary examination: streptomyces griseus, the producing organism for streptomycin, and S. clavuligerus. Biomass was washed twice with distilled water before freeze drying. This removed all soluble residues of medium constituents and cellular material solubilized during autoclaving. Cadmium binding was measured by atomic absorption spectrometry in an instrument fitted with a slotted quartz tube to enhance sensitivity. Langmuir plots of cadmium binding indicated that both types of biomass bound cadmium with sufficient avidity and had sufficient binding capacity for their use in subsequent work. Preliminary flotation experiments showed that both types of biomass could be readily recovered by flotation, before and after loading with cadmium. The importance of pH and biomass concentration to the biosorption process was investigated. Cadmium binding increased with increasing pH. Increasing biomass concentrations also increased cadmium removal, although the amount of cadmium bound per unit weight of biomass decreased. Consequently, even at pH values well below optimum, almost complete cadmium removal could be achieved at a sufficiently high biomass concentration. In general, new Streptomyces strains having the greatest resistance to cadmium toxicity exhibited the lowest cadmium binding capacities and low affinities for cadmium. All the strains with high affinities were susceptible to cadmium toxicity. Thus, a degree of selection for high capacity or high affinity could be made on the basis of cadmium resistance.

A project was set up which focused on the contacting of biomass with aqueous cadmium containing solutions and the elution of biosorbed cadmium from the biomass. Before use the biomass was washed with distilled water and then homogenized in order to maximize the surface area available for biosorption. Cadmium sulphate was used as the source of cadmium ions. The concentration of cadmium in solution was determined using a Unicam 929 atomic adsorption spectrometer. Factors investigated for their affect on biosorption of cadmium by the biomass included:
temperature - found to have no significant effect;
pH - biosorption was at its maximum between pH 6 and 10;
presence of celite - significant contribution at lower concentrations;
biomass density - 1 g/L represented the best compromise;
time - the process was virtually completed in around 15 minutes at 25 C;
initial cadmium concentration - with a biomass density of 1 g dry weight per litre and 5 mg cadmium/1 solution about 98% of the cadmium was adsorbed by the bacteria;
completion from other cations - preliminary results suggest that this could be a vital factor affecting the overall process;
presence of surfactants - at the envisaged concentrations there was no interference with cadmium biosorption;
recycle of used biomass - the cadmium removal capacity in fact increases after elution. There is, however, loss of biomass from the system in the liquid phase since the separation techniques are not 100% effective. This loss must be replaced with fresh biomass. Once the cadmium loaded bacteria had been separated from the liquid phase the metal was washed off the biomass by an elution procedure. This allowed the cadmium concentration to be increased to facilitate electrolytic removal of the metal. Elution using the fixed bed method consistently gave better than 97% elution of the biosorbed cadmium and produced at least a 20 fold increase in the cadmium concentration (ie high enough for electrolysis to be used to remove cadmium as metal powder).

Dead streptomyces has been used as a biosorbent for cadmium. 2 bacterial strains were studied: S. clavuligerus and S. griseus. Electrokinetic measurements showed increased negative values of 2 potential above pH 4; this observation related directly to flotation: above approximately -10 mV, diminishing biomass flotation was observed. A cationic surfactant was needed at about pH 7. 2 main flotation techniques using different bubble generation methods have been tested in the laboratory; they differed in bubble size and hydrodynamics. The use of ethanol in dispersed air flotation produced finer bubbles and hence quiescent conditions. The main parameters investigated batchwise were: the initial metal ion concentration, the quantity of biomass applied, the ionic strength, and the addition of flotation reagents. The solution pH was found to be a crucial parameter and the presence of a surfactant at an appropriate concentration was essential for good biomass flotation above pH 4. The temperature of the dispersion did not affect the flotation recoveries in the range examined. A thermodynamic study of equilibria by a computer programme was carried out; cadmium starts to precipitate out as hydroxide above pH 10, depending on the conditions. This was also found during ion flotation experiments without the use of any biomass; a precipitate flotation mechanism was proposed. A comparison of dispersed with dissolved air flotation was carried out. Both proved satisfactory. Cadmium elution tests of the floated biomass were carried out, either by pH adjustment to acidic values or the application of a chemical reagent. Biomass reflotation was shown to be possible and subsequently biosorbent recycling could be applied. A dispersed air flotation continuous unit has been constructed with a 2 L content, and using conditions optimized from the above bench scale experiments, the results and preliminary conclusions were fully confirmed. The cadmium loaded biomass recoveries obtained by flotation, in one stage, were over 85% under optimum conditions.

Criteria for choosing the electrolyte recovery of cadmium are quite restrictive. A simple salt solution was chosen with certain additives to assist the electrochemistry. The electrolysis system is a proprietary process based on a rotating cathode cell (R-C cell) producing metallic cadmium powder. This is a continuous flow cell operating at very high mass transfer and recovering the metal automatically outside the cell. The elution process produces a nominal 20 fold increase in concentration from the treated cadmium solution to the electrolyte. This means that the expected initial electrolyte cadmium concentration is in the range of 100 to 1000 mg/l. This is then reduced to a final level of the order of 1 to 10 mg/l by the R-C cell. Electrolysis have been successfully conducted with cadmium levels within the starting range going down to the final level at current densities between 2 KA m{-2} down to 0.2 KA m{-2} and electrolysis power consumptions of the order of 8 KWH/kgm of cadmium. The R-C Cell rig essentially consisted of 6 parts:
R-C Cell comprising tank, motor, drive shaft, cathode and cathode scraper assembly. On either side of the R-C is a sealed membrane anode compartment (SMAC) holding an oxygen evolution anode;
catholyte circuit supplying the catholyte liquor to the cathode and comprising task, pump, control valves, rotameter and sampling prints;
anolyte circuit supplying the SMACs with analyte liquor and comprising tank, pump, control valves and rotameter;
metal powder recovery circuit which collects metal powder produced on the R-C and dislodged by the scraper;
direct current circuit supplying current to the R-C Cell;
control panel consisting of 3 separate single phase 50 Hz 240 v power circuits each protected by an earth leakage circuit breaker and incorporating pump energizing switches and stop and start buttons, catholyte tank heater control unit, inverter system, energizing switch, controller and revolutions per minute read out for the R-C drive motor, and a digital volt meter and energizing switch for its power supply.
This is an application for funding for an innovative, collaborative project involving the Microbial Technology Group at Newcastle University, UK.; Electrolytic Services Limited (ETL), uK (an SME): and The Laboratory of Inorganic and General Chemistry, Aristotle University of Thessaloniki, Greece. It aims to develop a multidisciplinary approach to solving cadmium removal from Waste Water, involving microbiology, reactor design, flotation technology and electrochemistry. The succesful project will enable cadmium to be removed down to 10 parts per billion (ppb) and below, from industrial effluents (including the liquid effluent from ion-flotation processes), landfill leachates, mine drainage Waters, and other dilute solutions and recover the cadmium as elemental metal so that there is no cadmium waste disposal hazard.
The project will involve the following tasks:
(i) Isolation and selection of actinomycete bacteria having high binding capacity & good selectivity for cadmium, and measurement of their cadmium binding capabilities, using selective techniques developed at Newcastle; (ii) process development for contacting biomass and cadmium solutions; (iii) Flotation recovery of metal-loaded and re-cycled biomass using technology developed in Thessaloniki;
(iv) process development to achieve optimal bacterial growth morphology for the other stages of the integrated process;
(v) Elution oE cadmium and recycling of biomass;
(vi) Electrolytic recovery of cadmium from eluates of cadmium-loaded bacteria using the expertise of ETL;
(vii) Testing of the coordinated system.

Cadmium is a worldwide problem; adequate clean-up technology is not available at the moment. In this project the element is recovered as metallic cadmium. It can therefore be used directly in industry Without need for specialised cadmium Waste disposal techniques, thus offering considerable environmental protection. Successful development of the proposed cadmium removal and recovery process in the Community could


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Framlington Place
NE2 4HH Newcastle - Upon Tyne
United Kingdom

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