Removal of cyanide by woody plants

Hydrogen cyanide is a high volume production chemical that causes severe environmental problems. The toxicity of potassium cyanide (KCN) to basket willow trees (Salix viminalis) was tested. In aqueous solution, 2 mg CN l(-1) as KCN depressed the transpiration after 72 h about 50%. Trees exposed to 0.4 mg CN l(-1) in aqueous solution showed initially a depression of transpiration, but recovered. Doses of 8 and 20 mg CN l(-1) in aqueous solution were quickly mortal to the trees. At the end of the test, almost all cyanide had disappeared from the solutions. Levels of cyanide in plants were related to the toxicity, with no elevated levels of cyanide in plants exposed to 0.4 mg CN l(-1). Willows grown in sand survived 423.5 h irrigation with 20 mg CN l(-1). Willows grown in sand irrigated with 50 mg CN l(-1) died within a few days. The roots of the surviving willows were able to consume about 10 mg CN kg fresh weight(-1)h(-1). Vascular plants possess the enzymes beta-cyanoalanine synthase and beta-cyanoalanine hydrolase, which convert free cyanide to the amino acid asparagine. The in vivo capacity of woody plants (willow, poplar, elder, rose, birch) to remove cyanide was evaluated. Tests were performed with detached leaves and roots in KCN solutions of different concentrations. The highest removal capacity was obtained for basket willow hybrids (Salix viminalis x schwerinii). The Michaelis-Menten kinetics was determined. Realistic values of the half-saturation constant, K(M), were between 0.6 and 1.7 mg CN l(-1); the maximum metabolic capacity, v(max), was around 9.3 mg CN kg fresh weight(-1)h(-1). The removal of cyanide by plants might be useful in phytoremediation and treatment of wastewater from gold mining.     

Metabolism of cyanide by Chinese vegetation

Cyanide is a high-volume production chemical and the most commonly used leaching reagent for gold and silver extraction. Its environmental behavior and fate is of significant concern because it is a highly toxic compound. Vascular plants possess an enzyme system that detoxifies cyanide by converting it to the amino acid asparagine. This paper presents an investigation of the potential of Chinese vegetation to degrade cyanide. Detached leaves (1.5 g fresh weight) from 28 species of 23 families were kept in glass vessel with 100 ml of aqueous solution spiked with potassium cyanide at 23.5 degrees C for 28 h. Cyanide concentrations ranged from 0.83 to 1.0 CN mg l(-1). The disappearance of cyanide from the aqueous solution was analyzed spectrophotometrically. The fastest cyanide removal was by Chinese elder, Sambucus chinensis, with a removal capacity of 8.8 mg CN kg(-1) h(-1), followed by upright hedge-parsley (Torilis japonica) with a value of 7.5 mg CN kg(-1) h(-1). The lowest removal capacity had the snow-pine tree (Credrus deodara (Roxb.) Loud). Results from this investigation indicated that a wide range of plant species is able to efficiently metabolize cyanide. Therefore, cyanide elimination with plants seems to be a feasible option for cleaning soils and water contaminated by cyanide from gold and silver mines or from other sources.     

Ferrate(VI) oxidation of zinc-cyanide complex

Zinc-cyanide complexes are found in gold mining effluents and in metal finishing rinse water. The effect of Zn(II) on the oxidation of cyanide by ferrate(VI) (Fe(VI)O(4)(2-), Fe(VI)) was thus investigated by studying the kinetics of the reaction of Fe(VI) with cyanide present in a potassium salt of a zinc cyanide complex (K(2)Zn(CN)(4)) and in a mixture of Zn(II) and cyanide solutions as a function of pH (9.0-11.0). The rate-law for the oxidation of Zn(CN)(4)(2-) by Fe(VI) was found to be -d[Fe(VI)]/dt=k[Fe(VI)][Zn(CN)(4)(2-)](0.5). The rate constant, k, decreased with an increase in pH. The effect of temperature (15-45 degrees C) on the oxidation was studied at pH 9.0, which gave an activation energy of 45.7+/-1.5kJmol(-1). The cyanide oxidation rate decreased in the presence of the Zn(II) ions. However, Zn(II) ions had no effect on the cyanide removal efficiency by Fe(VI) and the stoichiometry of Fe(VI) to cyanide was approximately 1:1; similar to the stoichiometry in absence of Zn(II) ions. The destruction of cyanide by Fe(VI) resulted in cyanate. The experiments on removal of cyanide from rinse water using Fe(VI) demonstrated complete conversion of cyanide to cyanate.     

Phytotoxicity of Cyanide to Weeping Willow Trees

Background Cyanide is found predominantly in industrial effluents generated by metallurgical operations. It is an extremely toxic compound, so that problems and catastrophic accidents have recently occurred all around the globe. The goal of this study was to determine the toxicity of cyanide to a Chinese willow species, and to determine the removal capacity. Methods The toxicity of potassium cyanide (KCN) to weeping willow trees (Salix babylonica L.) was tested. The normalized, relative transpiration of the plants was used to determine the phytotoxicity of cyanide. The cyanide removal capacity of weeping willows was also determined. Results and Discussion In hydroponic solution, no chlorosis of leaves and only a small reduction in normalized relative transpiration was observed when weeping willows were exposed to low doses of cyanide (0.93 mg CN/L). Severe signs of toxicity were found for the treatment groups exposed to higher doses of cyanide (9.3 mg CN/L). Weeping willows grown in sandy soils survived the entire period (216 hours) without any toxic effect when irrigated with low doses of cyanide (3.72 mg CN/L). High doses of cyanide (18.6 mg CN/L) in irrigation water were fatal for the weeping willows within 216 hours. EC50 values for a 50% inhibition of the transpiration of the trees were estimated to be between 3.27 and 8.23 mg CN/L, depending on the duration of the exposure. Conclusions The results obtained for the Chinese willow species Salix babylonica were very similar to those obtained for the European species S. viminalis in earlier studies. Phytotoxic effects were only found at high doses of cyanide. A large proportion of applied cyanide was removed from the contaminated media in the presence of weeping willows. This gives rise to the conclusion that the metabolism of cyanide by weeping willows is possible. Recommendations and Outlook Cyanide elimination with trees seems to be a feasible option for cleaning soils and water contaminated with cyanide. A full-scale treatment has been installed in Denmark. For phytoremediation projects in China, weeping willow could be a suitable species. The tree can tolerate and remove cyanide, and it is a native Chinese species. Besides, the tree is of outstanding beauty and is planted as a common park tree in many parts of the world.     

Response of weeping willows to linear alkylbenzene sulfonate

Linear alkylbenzene sulfonate (LAS) is the most commonly used anionic surfactant in laundry detergents and cleaning agents. LAS compounds are found in surface waters and soils. The short-term acute toxicity of LAS to weeping willows (Salix babylonica L.) was investigated. Willow cuttings were grown in hydroponic solution spiked with LAS at 24.0+/-1 degrees C for 192 h. The normalized relative transpiration of plants was used to determine toxicity. Severe reduction of the transpiration was only found for high doses of LAS (240 mgl(-1)). Chlorophyll contents in leaves of treated plants varied with the dose of LAS, but there was no significant linear correlation. The activities of the enzymes superoxide dismutases (SOD), catalase (CAT), and peroxidase (POD) were quantified at the end of experiments. At higher concentrations of LAS (240 mgl(-1)), the activities of SOD and CAT were decreased. The correlation between the dose of LAS and the POD activity in leaf cells was the highest of all enzyme assays (R(2)=0.5). EC(50) values for a 50% inhibition of the transpiration of the trees were estimated to 374 mgl(-1) (72 h) and 166 mgl(-1) (192 h). Results from this experiment indicated that phytotoxic effects of LAS on willow trees are not expected for normal environmental conditions.     

Removal of Prussian blue from contaminated soil in the rhizosphere of cyanogenic plants

The fate of radiolabeled cyanide in soil was investigated during exposure to cyanogenic plant species, sorghum (Sorghum bicolor var. P721) and flax (Linum usitassimum var. Omega-Gold), in fully-contained growth chambers. Labeled cyanide was subject to microbial transformation, assimilation by plant roots, incorporation and biodegradation in plant tissue. For this study, (14)C-labeled cyanide was added to soil, and distribution of (14)C activity was assessed before plant establishment and after harvest. After 3 months of plant growth, 7% of the (14)C-labeled cyanide was converted to (14)CO(2) with sorghum and 6% with flax, compared with only 2% conversion in unplanted soil. A small amount of unaltered cyanide was shown to be accumulated by the plants (approximately 140 mg cyanide/kg plant or <0.1% of the total). Results from this experiment demonstrate the potential of cyanogenic plants for use in phytoremediation of cyanide-contaminated soil.     

富含活性氯与Al13水处理药剂对铜氰络合物去除效能

含重金属铜离子与氰离子（CN）的络合物广泛存在于电镀、冶金等工业废水中,是一种较难处理的污染物。富含活性氯和Al13聚合体的水处理药剂（PACC）兼具氧化和絮凝效能,在处理含重金属氰络合物（[Cu（CN）3]2-）废水方面具有良好的应用前景。研究PACC与[Cu（CN）3]2-的反应计量学、动力学,考察了pH、反应时间和投药量等影响因素,确定PACC的最佳工作参数。结果表明,PACC可同时实现对CN的氧化和对Cu2＋的絮凝,有效去除水中[cu（CN）3]2-使用PACC对[Cu（CN）3]2-的无害化处置过程分为2个阶段：CN-首先被氧化成氰酸根（OCN）;然后OCN-被进一步氧化并生成碳酸氢根和氮气,同时所释放的游离态铜离子被絮凝去除。这2个阶段反应的最佳pH分别为11和8,去除1tool[Cu（CN）3]2-的最佳投药量为9．35molCl2的PACC;在此条件下反应43min后,其出水中CN-和Cu2＋的浓度均达到排放标准（GB21900—2008）要求。     

The effect of temperature on the rate of cyanide metabolism of two woody plants

The response of cyanide metabolism rates of two woody plants to changes in temperature is investigated. Detached leaves (1.0 g fresh weight) from weeping willow (Salix babylonica L.) and Chinese elder (Sambucus chinensis Lindl.) were kept in glass vessels with 100ml of aqueous solution spiked with potassium cyanide for a maximum of 28 h. Ten different temperatures were used ranging from 11 degrees C to 32 degrees C. The disappearance of aqueous cyanide was analyzed spectrophotometrically. The cyanide removal rate of Chinese elder was higher than that of weeping willow at all temperatures. The highest cyanide removal rate for Chinese elder was found at 30 degrees C with a value of 12.6 mg CN kg(-1) h(-1), whereas the highest value of the weeping willow was 9.72 mg CN kg(-1) h(-1) at 32 degrees C. The temperature coefficient values, Q10, which are the ratio of removal rates at a 10 degree difference, were determined for Chinese elder and weeping willow to 1.84 and 2.09, respectively, indicating that the cyanide removal rate of weeping willow was much more susceptible to changes in temperature than that of the Chinese elder. In conclusion, changes in temperature have a substantial influence on the removal rate of cyanide by plants.     

Phytoremediation of atrazine by poplar trees: toxicity, uptake, and transformation

Toxicity, uptake, and transformation of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] by three species of poplar tree were assessed. Poplar cuttings were grown in sealed flasks with hydrophonic solutions and exposed to various concentrations of atrazine for a period of two weeks. Toxicity effects were evaluated by monitoring transpiration and measuring poplar cutting mass. Exposure to higher atrazine concentrations resulted in decrease of biomass and transpiration accompanied by leaf chlorosis and abscission. However, poplar cuttings exposed to lower concentrations of atrazine grew well and transpired at a constant rate during experiment periods. Poplar cuttings could take up, hydrolyze, and dealkylate atrazine to less toxic metabolites. Metabolism of atrazine occurred in roots, stems, and leaves and became more complete with increased residence time in tissue. These results suggest that phytoremediation is a viable approach to removing atrazine from contaminated water and should be considered for other contaminants.     

High levels of activity of bats at gold mining water bodies: implications for compliance with the International Cyanide Management Code

Wildlife and livestock are known to visit and interact with tailings dam and other wastewater impoundments at gold mines. When cyanide concentrations within these water bodies exceed a critical toxicity threshold, significant cyanide-related mortality events can occur in wildlife. Highly mobile taxa such as birds are particularly susceptible to cyanide toxicosis. Nocturnally active bats have similar access to uncovered wastewater impoundments as birds; however, cyanide toxicosis risks to bats remain ambiguous. This study investigated activity of bats in the airspace above two water bodies at an Australian gold mine, to assess the extent to which bats use these water bodies and hence are at potential risk of exposure to cyanide. Bat activity was present on most nights sampled during the 16-month survey period, although it was highly variable across nights and months. Therefore, despite the artificial nature of wastewater impoundments at gold mines, these structures present attractive habitats to bats. As tailings slurry and supernatant pooling within the tailings dam were consistently well below the industry protective concentration limit of 50 mg/L weak acid dissociable (WAD) cyanide, wastewater solutions stored within the tailings dam posed a minimal risk of cyanide toxicosis for wildlife, including bats. This study showed that passively recorded bat echolocation call data provides evidence of the presence and relative activity of bats above water bodies at mine sites. Furthermore, echolocation buzz calls recorded in the airspace directly above water provide indirect evidence of foraging and/or drinking. Both echolocation monitoring and systematic sampling of cyanide concentration in open wastewater impoundments can be incorporated into a gold mine risk-assessment model in order to evaluate the risk of bat exposure to cyanide. In relation to risk minimisation management practices, the most effective mechanism for preventing cyanide toxicosis to wildlife, including bats, is capping the concentration of cyanide in tailings discharged to open impoundments at 50 mg/L WAD.     

The dynamics of mercury around an artisanal and small-scale gold mining area,Camarines Norte,Philippines

Environmental Science and Pollution Research - To elucidate the dynamics of mercury emitted and released by artisanal and small-scale gold mining (ASGM) activity and to estimate its impact on the...     

富含活性氯与Al_(13)水处理药剂对铜氰络合物去除效能

含重金属铜离子与氰离子(CN)的络合物广泛存在于电镀、冶金等工业废水中,是一种较难处理的污染物。富含活性氯和Al13聚合体的水处理药剂(PACC)兼具氧化和絮凝效能,在处理含重金属氰络合物([Cu(CN)3]2-)废水方面具有良好的应用前景。研究PACC与[Cu(CN)3]2-的反应计量学、动力学,考察了pH、反应时间和投药量等影响因素,确定PACC的最佳工作参数。结果表明,PACC可同时实现对CN的氧化和对Cu2+的絮凝,有效去除水中[Cu(CN)3]2-。使用PACC对[Cu(CN)3]2-的无害化处置过程分为2个阶段:CN-首先被氧化成氰酸根(OCN-);然后OCN-被进一步氧化并生成碳酸氢根和氮气,同时所释放的游离态铜离子被絮凝去除。这2个阶段反应的最佳pH分别为11和8,去除1 mol[Cu(CN)3]2-的最佳投药量为9.35 mol Cl2的PACC;在此条件下反应43 min后,其出水中CN-和Cu2+的浓度均达到排放标准(GB21900-2008)要求。     

Levels of total cyanide and NaCl in surface waters adjacent to road salt storage facilities

Concentrations of Na, Cl and total cyanide (simple+complex forms of cyanide) in surface waters adjacent to uncovered, outdoor sand-salt storage facilities were monitored for the calendar year 1988. Runoff of deicing salt from these unprotected sand-salt piles resulted in Cl concentrations up to 13,500 mg liter(-1) in surrounding surface waters indicating substantial leaching of road salt from sand-salt storage lots. The use of sodium hexacyanoferrate (II) as an anti-caking agent in road salt resulted in up to 200 microg liter(-1) total cyanide (CN) in surface waters adjacent to sand-salt piles. Concentrations of Na and Cl were highest during the summer months whereas the concentrations of total CN were highest during the autumn months. The observed concentrations of total CN were less than the calculated maximum concentrations based on total CN content of pure road salt indicating that sodium hexacyanoferrate(II) was being adsorbed during overland flow onto soils and/or sediments of the wetlands. A laboratory adsorption study using five soils and a road salt solution showed that the soils adsorbed from 25 to 83% of the sodium hexacyanoferrate(II) present in the soil-road salt suspensions. The percentage of sodium hexacyanoferrate(II) adsorbed on soils increased with decreasing soil pH.     

Effects of nitrosation on the formation of cyanide in publicly owned treatment works secondary effluent.

Cyanide has been detected in the effluents of some publicly owned treatment works (POTWs) at levels exceeding the influent concentration. The presence of nitrite ion (NO2-) as a common constituent in domestic wastewater effluents may play an important role in the formation of cyanide through reaction with certain kinds of organic compounds, especially aromatic compounds. Laboratory studies with seven organic compounds (aniline. p-toluidine, phenol, 1,2,4-trihydroxybenzene, L-serine, glycine, and benzoic acid) revealed that cyanide can be formed by reaction of nitrite with some of these compounds. The most substantial free cyanide (HCN. CN-) production observed at 25 degrees C was 0.15 mg/L from reaction of 0.01 mM 1.2.4-trihydroxybenze with 5 mg/L nitrite for 72 hours. Substantial free cyanide formation was also observed at pH 2-4 in experiments with POTW effluents when reactive organics and nitrite were both added to wastewater. Formation of cyanide through nitrosation was strongly pH dependent, being most significant at low pH (2 to 4) and negligible at neutral-to-high pH. This result points to nitrous acid (HNO2) as being more reactive than the dissociated NO2- ion. The reaction of these nitrite species with organics also occurs in conventional analyses for total cyanide which involve distillation under strongly acidic conditions. Sufficient sample pretreatment with sulfamic acid at the time of sampling, not at the time of analysis. is highly recommended to prevent biasing analytical measurement of total cyanide in POTW effluents.     

Biological treatment of wastewater by selected aquatic plants

Pollutant-removal efficiency of certain macrophytes and algae, such as Eichhornia crassipes, Microcystis aeruginosa, Scenedesmus falcatus, Chlorella vulgaris and Chlamydomonas mirabilis, has been tested in laboratory conditions to evaluate their potential role in wastewater treatment. Sewage of Varanasi city, mixed with the effluents of about 1200 small-scale industries, was used for the tests. The investigation was performed in three stages i.e. a water hyacinth culture followed by an algal culture, and finally a second water hyacinth culture. For the first water hyacinth culture, 10 water hyacinth plants were grown in a tank of wastewater with 15 days' retention time. In the second stage, algal species were cultured in the treated wastewater for 5 days, whilst in the third stage, water hyacinth plants were again grown for further treatment of the wastewater for 9 days. This three-stage aquaculture resulted in very high reductions of BOD (96.9%), suspended solids (78.1%), total alkalinity (74.6%), PO(4)-P (89.2%), NO(3)-N (81.7%), acidity (73.3%), NH(4)-N (95.1%), COD (77.9%), hardness (68.6%) and coliform bacteria (99.2%). An increase in the concentration of dissolved oxygen (70%) was also observed.     

Effects of ozone exposure in open-top chambers on poplar (Populus nigra) and beech (Fagus sylvatica): a comparison

Rooted cuttings of poplar (Populus nigra) and seedlings of beech (Fagus sylvatica) were exposed to ozone in open-top chambers for one growing season. Three treatments were applied: charcoal-filtered (CF), non-filtered (NF) and non-filtered air plus 30 ppb (nl l(-1)) ozone (NF+). Extra ozone was only added on clear days, from 09:00 until 17:00-20:00. The AOT40s (accumulated exposure over a threshold of 40 ppb), calculated from April to September were 4055 ppb.h for the NF and 8880 ppb.h for the NF+ treatments. For poplar ozone exposure caused highly significant reductions in growth rate, light-saturated net CO(2) assimilation rate, stomatal conductance, F(v)/F(m) and chlorophyll content. The largest effects were observed in August at which time ozone concentrations were elevated. A reduction was noticed in new leaf production, while accelerated ageing and visible damage to leaves caused high leaf losses. For beech the responses were similar but less pronounced: ozone exposure resulted in non-significant growth reductions, slight changes in light-saturated photosynthesis and accelerated leaf abscission. The chlorophyll content of beech leaves was not affected by the ozone treatments. The results confirmed previous observations that fast-growing tree species, such as most poplar species and hybrids, are more sensitive and responsive to tropospheric ozone than slower-growing species, such as beech. The growth reductions observed and reported here for beech were within the range of those reported in relationship to the AOT40 (accumulated exposure over a threshold of 40 ppb) critical level for ozone.     

Cyanide Removal by Chinese Vegetation. Quantification of the Michaelis-Menten Kinetics (6 pp)

Background Little is known about metabolism rates of environmental chemicals by vegetation. A good model compound to study the variation of rates among plant species is cyanide. Vascular plants possess an enzyme system that detoxifies cyanide by converting it to the amino acid asparagine. Knowledge of the kinetic parameters, the half-saturation constant (Km) and the maximum metabolic capacity (vmax), is very useful for enzyme characterization and biochemical purposes. The goal of this study is to find the enzyme kinetics (KM and vmax) during cyanide metabolism in the presence of Chinese vegetation, to provide quantitative data for engineered phytoremediation, and to investigate the variation of metabolic rates of plants. Methods Detached leaves (1.0 g fresh weight) from 12 species out of 9 families were kept in glass vessels with 100 mL of aqueous solution spiked with potassium cyanide at 23°C for 28 h. Four different treatment concentrations of cyanide were used, ranging from 0.44 to 7.69 mg CN/L. The disappearance of cyanide from the aqueous solution was analyzed spectrophotometrically. Realistic values of the half-saturation constant (KM) and the maximum metabolic capacity (vmax) were estimated by a computer program using non-linear regression treatments. As a comparison, Lineweaver-Burk plots were also used to estimate the kinetic parameters. Results and Discussion The values obtained for KM and vmax varied with plant species. Using non-linear regression treatments, values of vmax and KM were found in a range between 6.68 and 21.91 mg CN/kg/h and 0.90 to 3.15 mg CN/L, respectively. The highest vmax was by Chinese elder (Sambucus chinensis), followed by upright hedge-parsley (Torilis japonica). The lowest vmax was demonstrated by the hybrid willow (Salix matssudana x alba). However, the highest KM was found in the water lily (Nymphea teragona), followed by the poplar (Populus deltoides Marsh). The lowest KM was demonstrated by corn (Zea mays L.). The values of vmax were normally distributed with a mean of 13 mg CN/kg/h. Conclusions Significant removal of cyanide from aqueous solution was observed in the presence of plant materials without phytotoxicity, even at high doses of cyanide. This gives rise to the conclusion that the Chinese plant species used in this study are all able to efficiently metabolize cyanide, although with different maximum metabolic capacities. A second conclusion is that the variation of metabolism rates between species is small. All these plants had a similar KM, indicating the same enzyme is active in all plants. Recommendations and Outlook Detoxification of cyanide with trees seems to be a feasible option for cleaning soils and water contaminated with cyanide. For phytoremediation projects, screening appropriate plant species adapted to local conditions should be seriously considered. More chemicals should be investigated to find common principles of the metabolism of environmental chemicals by plants.     

Assessment of the effluent quality from a gold mining industry in Ghana

The physical and chemical qualities of the process effluent and the tailings dam wastewater of AngloGold-Ashanti Limited, a gold mining company in Ghana, were studied from June to September, 2010. The process effluent from the gold extraction plant contains high amounts of suspended solids and is therefore highly turbid. Arsenic, copper and cyanide were identified as the major pollutants in the process effluent with average concentrations of 10.0, 3.1 and 21.6 mg?L^?1, respectively. Arsenic, copper, iron and free cyanide (CN^?) concentrations in the process effluent exceeded the Ghana EPA discharge limits; therefore, it is necessary to treat the process effluent before it can be discharged into the environment. Principal component analysis of the data indicated that the process effluent characteristics were influenced by the gold extraction process as well as the nature of the gold-bearing ore processed. No significant correlation was observed between the wastewater characteristics themselves, except for the dissolved oxygen and the biochemical oxygen demand. The process effluent is fed to the Sansu tailings dam, which removes 99.9 % of the total suspended solids and 99.7 % of the turbidity; but copper, arsenic and cyanide concentrations were still high. The effluent produced can be classified as inorganic with a high load of non-biodegradable compounds. It was noted that, though the Sansu tailings dam stores the polluted effluent from the gold extraction plant, there will still be serious environmental problems in the event of failure of the dam.     

Electrogenerative gold recovery from cyanide solutions using a flow-through cell with activated reticulated vitreous carbon

An electrogenerative flow-through reactor with an activated reticulated vitreous carbon cathode was developed. The influence of palladium-tin activation of the cathode towards gold deposition was studied by cyclic voltammetry. The reactor proved to be efficient in recovering more than 99% of gold within 4 h of operation. The performance of the reactor was evaluated with initial gold concentrations of 10, 100 and 500 mg L-1 and various electrolyte flow rates. Gold recovery was found to be strongly dependent on electrolyte flow rate and initial gold concentration in the cyanide solution under the experimental conditions used.     

Biological treatment of a synthetic gold milling effluent

This paper reports on biological sludge acclimatisation and the results concerning the removal of free cyanide, thiocyanate and metallocyanides (copper, iron and zinc) from a synthetic gold milling effluent. The experiments were carried out in a continuous bench-scale bioreactor, and the experimental set-up consisted of two identical units, one of which served as control. The acclimatisation of the biomass was based on a stepwise procedure, in which the proportion of synthetic solution in the influent was gradually increased. The reactors were fed with a mixture of synthetic effluent and sewage, and the treatment efficiency was evaluated through the monitoring of the following parameters: chemical oxygen demand (COD), free cyanide, thiocyanate, copper, iron and zinc concentrations. A well adapted microbial consortium was obtained at the end of the acclimatisation period, which was able to remove more than 95% of free cyanide, thiocyanate, copper and zinc, originally found in the influent. These removal efficiencies were obtained when the reactor was operated with a hydraulic retention time (HRT) of about 8 h. The performance results of experiments carried out with lower HRT (5 h) and higher dissolved oxygen (DO) concentration (6.5 mg litre(-1)) are also presented and discussed in this study.