Free Cyanide Research Articles

NUTRITIONAL AND ANTINUTRITIONAL ASSESSMENT OF SOME UNDERUTILIZED CORMS, RHIZOMES AND TUBERS

<p><strong>Background. </strong>With ever-increasing populace pressure and rapid depletion of natural resources, it has become exceptionally important to diversify the present time agriculture with the cultivation of some wild varieties of tubers, rhizomes and corms in order to meet various human nutrient needs. However, information regarding the nutritional and antinutritional composition is meager. <strong>Objective.</strong> To study the chemical composition and antinutritional factors of the wild edible corms, rhizomes and tubers consumed by the Palliyars and Kanikkar tribes living in South Eastern Slopes of Western Ghats, Tamil Nadu<strong> Methodology</strong>. The wild edible corms of <em>Alocasia macrorrhiza</em>, <em>Amorphophallus paeoniifolius</em> var <em>campanulatus</em>,<em> Amorphophallus sylvaticus</em>,<em> Colocasia esculenta</em>,<em> Xanthosoma sagittifolium</em>, <em>Xanthosoma violaceum</em>, rhizomes of <em>Canna indica</em>,<em> Maranta arundinacea</em> and tubers of <em>Asparagus racemosus</em>,<em> Nymphaea pubescens</em> and<em> Nymphaea rubra</em> were analysed for proximate and mineral composition, starch, vitamins like niacin, ascorbic acid and antinutritional factors like total free phenolics, tannins, hydrogen cyanide, total oxalate, amylase and trypsin inhibitor activities were quantified. Also, <em>in vitro</em> protein and starch digestibility were assessed. <strong>Results.</strong> The rhizome of <em>Maranta arundinacea</em> and tubers of <em>Nymphaea pubescens </em>contain high quantity of crude proteins. The tubers of <em>Asparagus racemosus</em> contained higher amount of crude lipids. The corms of <em>Alocasia macrorrhiza, Colocasia esculenta, Xanthosoma sagittifolium</em> and <em>Xanthosoma violaceum</em> appeared to have a higher level of potassium content compared to recommended Dietary allowances (RDA) for infants, children and adults. The corms of <em>Alocasia macrorrhiza, Amorphophallus sylvaticus, Xanthosoma violaceum</em> and rhizomes of <em>Maranta arundinacea</em> were found to contain more starch. All the investigated samples had low <em>in vitro</em> protein digestibility. <strong>Implications.</strong> The present result highlights the potentiality of these underground plant parts as source of unconventional foods. Being wild, they also are easily accessible and cheaper. Studies on nutritional value of wild plant food are of extensive importance since it may help to recognize long forgotten food resources. <strong>Conclusion. </strong>Most of the wild edible corms, rhizomes and tubers were found to be a good source of protein, lipid, total dietary fiber, starch, vitamins and minerals. All the investigated samples exhibited variations in the levels of the total free phenolics, tannins, hydrogen cyanide, total oxalate, amylase and trypsin inhibitors</p>

Niobium-Titanium-Based Photocatalysts: Its Potentials for Free Cyanide Oxidation in Residual Aqueous Effluent.

Organic compounds are employed as additives to increase the dissolution speed of gold, in concentrations around 1 g/L when using cyanidation, thereby forming a residual aqueous effluent with high amounts of free cyanides and organic compounds, which generate metallic complexes difficult to degrade. To increase the photodegradation efficiency, promising niobium and titanium porous materials are proposed as photocatalysts, due to their role in simultaneous oxidation and reduction reactions. In the process of cyanide oxidation, NbO5 0.3H2O was doped with titanium oxalate (IV) of 0.5, 1, and 1.5%; and HTiNbO5 were synthesized, from the mixture of NbO5 with TiO2 Degussa-P25, by coprecipitation, impregnation, and solid state. The determination of its elemental composition, morphological and textural properties were carried out by using various XRD techniques, Raman spectroscopy, SEM/EDS and acidity by pyridine. The experiments of photocatalytic oxidation of cyanide used one semibatch reactor with ultraviolet irradiation 125 W in a pH range of 9.5–12. The catalyst with the highest percentage of degradation was HTiNbO5 93.7%, which is attributed to the microstructure of the double layer and Lewis acidity sites, followed by NbTi-1% 92.9% and the Nb2O5.3H2O 82.4%, being the majority product cyanate, proposing its mechanism of reaction. Characterization experiments indicated Nb-O-Ti bridges that have been associated with the control of redox properties of the niobium species and Ti-O-Nb = O, which could be generating a greater number of e–H +pairs, increasing the photocatalytic activity. It is considered that the method of synthesis has a strong influence in changing the morphology of the particles such as porosity, specific surface and factors such as the acidity of niobium–based catalysts, which are important to achieving efficiency in degradation. Niobium-Titanium photocatalysts proved to be an excellent new breakthrough in Advanced Oxidation Technologies (AOT), to eliminate cyanide in wastewater from mining activities.

Photocatalytic degradation of ferricyanide as synthetic gold mining wastewater using TiO2 assisted by H2O2

Abstract Ferricyanide Fe(CN) 6 3- is one of the most stable cyanometallic complexes present in the gold mining effluents. This cyanocomplex is hard to degrade by natural attenuation and generates a negative impact on aquatic environments. Although free cyanide (CN-) can be obtained by acidifying the solution, the CN- is lethal for all forms of life. The oxidation of Fe(CN) 6 3- in a typical photocatalitic system was evaluated with the addition of H2O2. To verify the degradation, chemical parameters, such as free cyanide, the formation of ammonia, nitrate, and total iron were analyzed at the end of the process. Different parameters were evaluated to analyze the behavior of the degradation: 1. dark stage adsorption using the catalyst, 2. the TiO2 dosage, 3. Addition of H2O2, 4. UV radiation power (120 and 200W) and finally a test of TiO2 with solar radiation. The photolysis effect from a ferricyanide solution at 100 mg L-1 at alkaline pH 13, showed that the complex studied is highly stable since under UV irradiation conditions (l> 300 nm), a low rate of dissociation was observed. After 24h of irradiation, the cyanocomplex was under 20%, whereas degradations up to 70% were obtained in a heterogeneous photocatalysis system with TiO2. The best result was achieved with the H2O2 and TiO2 photocatalytic system, and the stoichiometric concentration was about 2.5 times less than the peroxide used in the gold mining industry, reaching 83% degradation. The photocatalytic process obtained less toxic byproducts than the original synthetic ferricyanide used as mining wastewater.

A mechanism of metastable sulfur speciation and the adsorption on a gold surface in the presence of sulfidic ore and lead in cyanide medium

Metallic gold or gold minerals have high affinity towards aqueous sulfur species during gold leaching process in cyanide solutions, thus the gold surface can often become passivated by the reaction between the different sulfur species and gold. During the cyanide leaching of sulfidic gold ores, the sulfide minerals dissolve into the leach solution and produce metastable sulfur species, such as sulfide, bisulfide, polysulfide, elemental sulfur, and thiocyanate. Most of the above sulfur species can directly passivate the gold surface, and significantly reduce the gold leaching efficiency.This paper pertains to studying the sulfur species formation in cyanide solutions and their adsorption chemistry and mechanism on gold surface. It was found that the lead salts, often added to the cyanidation process to improve the gold leaching efficiency, act as a sulfide oxidant and accelerates its oxidation to different metastable sulfur species. It was shown by electrochemical linear sweep voltammetry (LSV) tests that the gold oxidation current density, i.e. oxidation kinetics, was decreased 32%, from 2.20 to 1.50 mA/cm2, in the presence of a high sulfide ore by increasing the amount of lead salt from 0 to 50 mg/L at 300 mg/L free cyanide (300 mg/L CN). Monosulfide, disulfide and polysulfide sulfur species were detected on the gold surface, by X-ray photoelectron spectroscopy (XPS) method, confirming the adsorption of different sulfur species on the gold surface. At high concentration of added lead, 50 mg/L Pb2+, even elemental sulfur was shown to adsorb on the gold surface. The formation of different sulfur species was explored by thermodynamic modeling, which showed very good agreement with the results of the XPS study. This study helps to develop a better insight on the mechanism of gold surface passivation caused by sulfur species and the findings help to reduce gold passivation by better control of the process condition.

Investigation of mercury cyanide adsorption from synthetic wastewater aqueous solution on granular activated carbon

Mercury (Hg) is one of the most harmful metals to human health. Combined with cyanide (CN−), Hg forms a stable complex (Hg(CN)2) that is difficult to remove from solution. Artisanal gold mining effluents containing mercury cyanide complexes are discharged into rivers and streams, which can be potentially bioavailable to aquatic organisms. In this study, activated carbon adsorption was evaluated for its capacity to remove mercury cyanide from solution. A solution with Hg(CN)2 and free cyanide was prepared at pH 12, corresponding to 1 mg/L Hg2+ and 0.5 mg/L CN. Analysis in batch mode showed that 180 min were required to achieve adsorption equilibrium. The adsorbent dosage was fixed at 0.1 g of activated carbon, which represented an adsorption capacity of 0.14 mg/g, equivalent to adsorption of 81 % Hg(CN)2. The free cyanide concentration did not affect the adsorption of mercury cyanide. Experimental data indicated that the adsorption follows pseudo-second order kinetics and equilibrium results corresponded to the Freundlich adsorption isotherm.

Concurrent detection and treatment of cyanide-contaminated water using mechanosynthesized receptors.

The development of receptors that can detect as well as treat cyanide ions in aqueous samples is indispensable for environmental protection. Herein, we present the bulk solvent-free and instant green synthesis of a series of turn-on fluorimetric probes that can specifically detect the deadly poison cyanide among various anions and metal ions in water. Selective recognition of cyanide by the mechanosynthesized compounds is even observable by the naked eyes, which remained unaffected in the presence of various challenging species. NMR spectroscopic investigation supports the chemodosimetric sensing of cyanide by the receptors. A remarkable 55-83 fold fluorescence enhancement by the probes enabled us to reach a limit of detection (LOD) in the range of 8-26 ppb, well below the permissible limit of cyanide in drinking water. Being minuscule soluble in water, cyanide treatment studies with the ionophores showed greater than 99% reduction in the free cyanide concentration after three consecutive cycles of operation. Furthermore, the compounds can be used as sensitive probes for the estimation of cyanide in human blood serum in physiological conditions. Overall, the results presented in this article will certainly find great use in the area of cyanide pollution with regard to simultaneous sensing and treatment of free cyanide, which is heretofore unprecedented.

Microbial biodiesel production: novel method of utilizing sago wastewater for lipid production using oleaginous yeast, Candida tropicalis ASY1

A study was undertaken to determine the physico-chemical characteristicsof sago wastewater (SWW) from various sago industries in Salem andNamakkal districts of India. Besides, different SWW (raw wastewater,aerobic wastewater, treated wastewater and starch wastewater) from withinthe industry were evaluated. The results concluded that effluents from thisregion are whitish and greyish brown in color with majority supra-colloidalor settable suspended solids with high TS, TSS, and TDS. The ammoniacal,nitrate, and nitrite nitrogen ranged from 2.7 to 6.5, 4.5 to 19.1, and 0.4to 2.8 mg.L-1. The total nitrogen content of SWW samples was ranged from180- 650 mg.L-1. The phosphate level of SWW was very high (640-1096 mg.L-1). The COD and BOD content of SWW from all the industries ranged from2560 to 3200 mg.L-1 and 34000 to 76000 mg.L-1 indicates its high organicnature. SWW effluents are acidic in nature (pH 4.3-5.2). The free cyanide(CN) content was ranged from 1.2 to 5.2 mg.L-1. The treated wastewater hadneutral pH (pH - 6.2), and had a reduced level of EC, starch content, TSS,COD and BOD.

Biodegradation of cyanide using recombinant Escherichia coli expressing Bacillus pumilus cyanide dihydratase

Despite its high toxicity, cyanide is used in several industrial processes, and as a result, large volumes of cyanide wastewater need to be treated prior to discharge. Enzymatic degradation of industrial cyanide wastewater by cyanide dihydratase, which is capable of converting cyanide to ammonia and formate, is an attractive alternative to conventional chemical methods of cyanide decontamination. However, the main impediment to the use of this enzyme for the biodegradation of cyanide is the intolerance to the alkaline pH at which cyanide waste is kept and its low thermoresistance. In the present study, the catalytic properties of whole E. coli cells overexpressing a cyanide dihydratase gene from B. pumilus were compared to those of the purified enzyme under conditions similar to those found in industrial cyanide wastewater. In addition, the capacity of whole cells to degrade free cyanide in contaminated wastewater resulting from the gold mining process was also determined. The characteristics of intracellular enzyme, relative to purified enzyme, included increased thermostability, as well as greater tolerance to heavy metals and to a lesser extent pH. On the other hand, significant enzymatic degradation (70%) of free cyanide in the industrial sample was achieved only after dilution. Nevertheless, the increased thermostability observed for intracellular CynD suggest that whole cells of E. coli overexpressing CynD are suited for process that operate at elevated temperatures, a limitation observed for the purified enzyme.

The role of a chemical loop in removal of hazardous contaminants from coke oven wastewater during its treatment

AbstractCoke oven liquor is one of the most contaminated liquid streams generated by the coal processing industry, thus its proper treatment and utilization is crucial for sustainable and environmentally neutral plant operation. The conventional wastewater treatment process comprises of chemical and biological processes. Within the current research the detailed role of chemical treatment is described. Commercially available iron-based coagulants (PIX100, PIX100COP, PIX113, PIX116) were tested to understand their removal efficiency and impact on the stream parameters. The influence of iron dose in the range of 300-500 mgFe/L on the process performance was also examined.It was found that the main role of chemical treatment was to bind toxicants harmful to activated sludge microorganisms, i.e. free and complex cyanides and sulphides. Among the tested iron-based conventional coagulants ferrous salts were more efficient than ferric salts. It was also observed that efficiency of the process strongly depended on wastewater properties (especially in regard to pH, which should be in the range of 9-10) and the coagulant selection needed to be done individually for a given wastewater type. The removal rates of particular contaminants were diversified and for free cyanides, complex cyanides and sulphides they were in the range of 23-91%, -156-77% and -357-98%, respectively. The expected, simultaneous removal of chemical oxygen demand (COD) during the treatment was not observed and even the parameter value increased after the process due to probable formation of compounds less vulnerable to oxidation.

Kinetics of Thiocyanate Formation by Reaction of Cyanide and Its Iron Complexes with Thiosulfate

Reactions between cyanide and compounds, which contain S–S bonds, in aqueous media result in formation of thiocyanate. In this work, we studied the kinetics of reactions of thiosulfate with free cyanide and its complexes under environmental conditions. Rates of reactions between cyanide species and thiosulfate decrease in the following order: CN− > HCN > [Fe(CN)6]3− > [Fe(CN)6]4−. However, at neutral and slightly acidic pH range, reaction of thiosulfate with iron-cyanide complexes outcompetes its reaction with free cyanide, which exists in equilibrium with complexed cyanide. At environmentally relevant conditions, the characteristic time of reaction between free cyanide and thiosulfate was found to be tens of thousands of years, while for iron-cyanide complexes it was found to be hundreds to millions of years. Examples of application of kinetic parameters for calculation of rates of cyanide consumption in industrial (coke oven wastewater) and non-polluted natural aquatic system (Delaware Great Marsh) are provided.

Sustainable Approach to Eradicate the Inhibitory Effect of Free-Cyanide on Simultaneous Nitrification and Aerobic Denitrification during Wastewater Treatment

Simultaneous nitrification and aerobic denitrification (SNaD) is a preferred method for single stage total nitrogen (TN) removal, which was recently proposed to improve wastewater treatment plant design. However, SNaD processes are prone to inhibition by toxicant loading with free cyanide (FCN) possessing the highest inhibitory effect on such processes, rendering these processes ineffective. Despite the best efforts of regulators to limit toxicant disposal into municipal wastewater sewage systems (MWSSs), FCN still enters MWSSs through various pathways; hence, it has been suggested that FCN resistant or tolerant microorganisms be utilized for processes such as SNaD. To mitigate toxicant loading, organisms in SNaD have been observed to adopt a diauxic growth strategy to sequentially degrade FCN during primary growth and subsequently degrade TN during the secondary growth phase. However, FCN degrading microorganisms are not widely used for SNaD in MWSSs due to inadequate application of suitable microorganisms (Chromobacterium violaceum, Pseudomonas aeruginosa, Thiobacillus denitrificans, Rhodospirillum palustris, Klebsiella pneumoniae, and Alcaligenes faecalis) commonly used in single-stage SNaD. This review expatiates the biological remedial strategy to limit the inhibition of SNaD by FCN through the use of FCN degrading or resistant microorganisms. The use of FCN degrading or resistant microorganisms for SNaD is a cost-effective method compared to the use of other methods of FCN removal prior to TN removal, as they involve multi-stage systems (as currently observed in MWSSs). The use of FCN degrading microorganisms, particularly when used as a consortium, presents a promising and sustainable resolution to mitigate inhibitory effects of FCN in SNaD.

Cu adsorption behaviours onto chelating resins from glycine-cyanide solutions: Isotherms, kinetics and regeneration studies

A method is presented to remove and recover copper from copper-gold glycinate solutions using an iminodiacetic functionalized resin with subsequent regeneration. The adsorption/desorption approach was evaluated for a glycine leaching process which was shown to be effective for the dissolution of gold and copper from different gold-copper ores and concentrates. In this process, glycine is the lixiviant in the presence of very low cyanide concentration as a synergistic reagent. The copper species in glycine-cyanide process are mainly present as cupric glycinate and a low concentration of cuprous dicyanide and no free cyanide. While cupric glycinate does not load well onto activated carbon, cuprous dicyanide is problematic due to its high carbon affinity and co-loading with gold onto activated carbon, thereby requiring the removal of most of the copper prior to the gold recovery. In this study, the use of ion-exchange resins as one of the possible downstream recovery processes for gold and copper from glycine-cyanide solutions was tested. The isotherms and kinetics of gold and copper (cupric copper (Cu2+)) adsorb onto the chelating resin (Puromet MTS9300) from their alkaline glycinate and cyanide complexes were studied. Results show that the [CNT−]: [CuT] molar ratio has significant effects on the adsorption of CuT, due to the change in Cu2+/Cu+ ratio in the solutions, where [CN−T] is the total cyanide in the system (free cyanide being zero). The results show that the chelating resin is selectively adsorbed cupric (Cu2+) over cuprous (Cu+) ions. The adsorption results were fitted to both the Freundlich and Langmuir isotherm models, while the Langmuir model shows a better correlation. The adsorption kinetic data of Cu2+ is well fitted to the pseudo-second-order model. Alkaline glycine solutions in the presence of NaCl was selected as the main eluent for the resin elution study. The multi-cycle adsorption/desorption tests showed that the resin adsorption and regeneration efficiencies are not significantly changed after five cycles of adsorption/desorption. Microscopic visual analysis by using scanning electron microscopy (SEM) showed that there is no significant change in shape and size of the resins, although more cracks were observed after the 5 cycles of adsorption/desorption.

Determination and detoxification of cyanide in gold mine tailings: A review.

Cyanide is among the most toxic chemicals widely employed in the cyanidation process to leach precious minerals, such as gold and silver, by the minerals processing companies worldwide. This present article reviews the determination and detoxification of cyanide found in gold mine tailings. Most of the cyanide remains in the solution or the slurries after the cyanidation process. The cyanide species in the gold tailings are classified as free cyanide, weak acid dissociation, and metallocyanide complexes. Several methods, such as colorimetric, titrimetric, and electrochemical, have been developed to determine cyanide concentrations in gold mine effluents. Application of physical, natural, biological, and chemical methods to detoxify cyanide to a permissible limit (50 mg L-1) can be achieved when the chemical compositions of cyanide (type of species) present in the tailings are known. The levels of cyanide concentration determine the impact it will have on the environment.

ANÁLISE DA ESTABILIDADE DE CIANETO LIVRE EM MANIPUEIRA PARA UTILIZAÇÃO EM MANEJOS FITOSSANITÁRIOS DE PRAGAS E DOENÇAS

A manipueira, resíduo líquido extraído da mandioca (Manihot esculenta L.) durante o processo de fabricação de farinha e (ou) amido, apresenta um glicosídeo tóxico denominado linamarina, do qual se origina o ácido cianídrico (HCN). As características químicas e orgânicas da manipueira possibilitam a sua utilização no manejo fitossanitário de plantas agrícolas, objetivando reduzir as populações de patógenos a níveis inferiores aqueles capazes de causar perdas. No entanto, apesar do avanço nas pesquisas, pouco se conhece a respeito da estabilidade do ácido cianídrico na manipueira após sua extração. Objetivou-se com o presente estudo avaliar a estabilidade e permanência do ácido cianídrico (HCN) na composição da manipueira, visando auxiliar pesquisadores e produtores no uso mais eficiente e racional deste resíduo. Conclui-se que a manipueira apresenta variável estabilidade e contínua permanência do ácido cianídrico (HCN) em sua composição, dentro do período avaliado, independente da forma de armazenamento.

Cyanide Degradation from Mining Effluent Using Two Reagents: Sodium Metabisulphite and the Metabisulphite Mixture with Hydrogen Peroxide

Cyanide ion (CN-) is widely used in different industrial operations, such as jewelry, steel manufacture, gold and silver extraction and electroplating. However, industrial emissions containing the CN- ion have to be treated to comply with environmental regulations. This research aimed to degrade free cyanide (CNL) present in the tailing of a metallurgical plant that processes gold-bearing ores and uses sodium cyanide (NaCN) as a leaching reagent. Sodium metabisulfite (Na2S2O5) and sodium metabisulfite with hydrogen peroxide (Na2S2O5 + H2O2) were used as oxidizing agents. To evaluate the effect of the factors, we used a factorial design with three independent variables: stirring time, reagent excess percentage, type of reagent and a dependent variable: CNL degradation (mg/L). According to the analysis of variance (ANOVA), the variables influenced significantly CNL degradation, being the most relevant the reagent excess percentage and according to the results, the maximum CNL degradation was 97.67% when 400% of Na2S2O5excess was added with 4 hours of stirring.

Destruction of Cyanide and Removal of Copper from Waste Printed Circuit Boards Leach Solution Using Electro-Generated Hypochlorite Followed by Magnetite Adsorption

The removal of the cyanide and copper (Cu) from the waste printed circuit boards (WPCBs) cyanide leach solution through the alkaline chlorination using electro-generated hypochlorite (NaOCl) followed by magnetite (Fe3O4) adsorption is investigated. The efficiency of the destruction of cyanide and precipitation of Cu was increased with increasing the concentration of free available chlorine in NaOCl. More than 99% of free cyanide and 76% of Cu were removed under the following conditions: concentration of chlorine in electro-generated NaOCl, 5.2 g/L; volume ratio of NaOCl/leach solution, 1; pH, ~9.8; ambient temperature for 12 h. Then, magnetite adsorption for selective removal of remaining Cu (50.5 mg/L) was selected and more than 99% of copper ion was successfully removed with dosage 10g/100mL, shaking speed 150 rpm within 30 min. The results revealed that the alkaline chlorination using electro-generated NaOCl followed by magnetite adsorption could completely remove the cyanide and Cu, remaining Au in the final solution.

Thermal remediation of cyanide-contaminated soils：process optimization and mechanistic study

Site soils with persistent cyanide compounds (primarily iron-cyanide complex) pose potential hazards to the environment and require remediation before redevelopment. This study evaluated the possibility of thermal treatment on remediation of cyanide-contaminated soils via batch heating experiments spanning a wide temperature range (200–500 °C). The change with operation variables of total cyanide and some reaction intermediates (e.g. CN−) was analyzed in order to elucidate the optimal variables that guarantee cyanide removal while generating no hazardous byproducts. Temperature, heating time and cyanide species have been found to be important parameters influencing removal/destruction of cyanide in soils. For soils bearing K3[Fe(CN)6] and K4[Fe(CN)6], a removal efficiency of >99.9% can be obtained with temperatures over 350 °C at 1 h, while for samples bearing Fe4[Fe(CN)6]3, a higher temperature (>450 °C) is needed to obtain an equivalent efficiency. During heating, the iron-cyanide complexes decomposed, releasing highly toxic free cyanides, which will subsequently be oxidized. However, a small percentage of free cyanide can always be detected as a result of incomplete oxidation, thus caution should be taken to minimize the accumulation of free cyanide during thermal treatment.

An Alkaline Glycine-Based Leach Process of Base and Precious Metals from Powdered Waste Printed Circuit Boards

Electronic waste (E-waste) is accumulating rapidly globally and pose a significant environmental challenge. One of the ways to cover the cost of waste processing (in addition to reducing the costs associated with landfill) is through recovery of metals. In addition, toxic and dangerous metals can and must be removed prior to repurposing, incineration or pyrolysis of the plastic substrates. E-waste is usually either transported to landfills or processed by pyrometallurgical and hydrometallurgical processes. Recently, a number of hydrometallurgical approaches have been considered in metals recovery from different electronic components. In this study, glycine (amino acetic acid) or its salts is considered as a lixiviant in an alkaline environment for base and precious metals recovery from shredded and ground printed circuit boards (PCBs). It was found that alkaline glycine solutions selectively dissolve copper, zinc, and lead over precious metals. Gold and silver were then recovered in a subsequent leaching step using glycine and small amounts of cyanide (at starvation levels, implying no free cyanide is present). The leach system remains alkaline throughout both stages of processing. In the two-stage glycine leaching system, gold, silver, zinc, lead and copper recoveries were 92.1%, 85.3%, 98.5%, 89.8%, and 99.1% respectively. The recoveries of precious and base metals by direct cyanidation, single stage glycine–cyanide leaching, and ammonia leaching were lower than the recoveries of these metals using the two-stage glycine and glycine–cyanide systems.

Performance of various cyanide degrading bacteria on the biodegradation of free cyanide in water

This study reports on the biodegradation of free cyanide (FCN) by cyanide degrading bacteria (CDB) that were isolated from mining wastewater and thiocyanate containing wastewater. The performance of these isolates was compared to cryopreserved CDBs that were used in previous studies. The performance of the isolates to degrade FCN was studied in batch cultures. It was observed that the CDB from the thiocyanate wastewater showed higher biodegradation rates (2.114 g CN−. L−1.O.D600 nm−1.h−1) compared to the isolates from the mining wastewater. The isolates from the cryopreserved CDBs and from the mining wastewater achieved a biodegradation rate of 1.285 g CN− L−1.O.D600 nm−1.h−1 and 1.209 g CN−.L−1.O.D600 nm−1.h−1, respectively. This study demonstrated that the source of the organisms plays a significant role on FCN biodegradation.

The effect of thermal and acid treatment of kaolin on its ability for cyanide removal from aqueous solutions

The removal of free cyanide from aqueous solutions was investigated by kaolin, calcined kaolin (metakaolin), and acid-treated metakaolin. The kaolin in raw form had no ability for cyanide adsorption, but calcination and acid treatment significantly improve its adsorptive properties. The best calcination conditions were determined to be at temperature of 600 °C and calcination time of 1 h. The scanning electron microscope (SEM), wavelength-dispersive X-ray (WDX), X-ray fluorescence (XRF), and X-ray diffraction (XRD) analysis methods were used to explain the adsorptive properties of the adsorbents. The loading capacity of metakaolin increased from 0.66 to 5.32 mg/g, and the loading capacity of acid-treated metakaolin increased from 1.33 to 10.67 mg/g by increasing the initial cyanide concentration from 25 to 531 mg/L. The kinetics of cyanide adsorption onto both adsorbents increases by increasing the initial cyanide concentration (25–531 mg/L) and solution temperature (30–50 °C). The equilibrium and kinetic data on both adsorbents are best fitted by the Freundlich isotherm and pseudo-second-order kinetic models, respectively. The thermodynamic study showed that the cyanide adsorption onto both adsorbents is endothermic and spontaneous in nature. The activation energy for the cyanide adsorption onto metakaolin and acid-treated metakaolin was determined to be 16.68 and 5.79 kJ/mol which indicates that the adsorption of cyanide ions onto metakaolin is chemisorption and onto the acid-treated metakaolin is a physicochemical process.