Cyanidation of mercury-rich tailings in artisanal and small-scale gold mining: identifying strategies to manage environmental risks in Southern Ecuador

In many countries, such as Brazil, Colombia, Ecuador, Indonesia, Venezuela and Zimbabwe, amalgamated tailings are leached with cyanide to recover remaining gold. This paper describes a recently completed study conducted at seven gold processing centers in Portovelo-Zaruma, Southern Ecuador, which involved consultation with local miners. The objective of the study was to understand the behaviour of mercury (with a focus on mercury loss) in artisanal gold mining operations through the evaluation of two cyanidation processes (Merrill-Crowe and Carbon-in-pulp), using a participatory approach. In order to assess the kinetics of mercury dissolution in cyanide, a bottle roll test was conducted in the laboratory. In the Merrill-Crowe cyanidation process, an average of 24.2% of metallic mercury was determined to be trapped at the bottom of the agitation tanks; 33.1% of mercury is lost in association with solid material and 11.7% is lost in solution. Approximately 31.0% of mercury in solution is sent to the zinc precipitation cells from which 27.8% is precipitated on the zinc shavings, with 3.23% remaining in solution. The mercury precipitated on to the zinc is lost to the atmosphere when the shavings are burned at 900 °C. In the Carbon-in-pulp (CIP) leaching system, 11.2% of the mercury is lost with the solid tailings; 31.6% of the mercury is associated with fine particles in suspension and 50.8% is likely dissolved. About 2.68% is trapped at the bottom of the tank and 3.72% is absorbed by the activated carbon. The bottle roll test revealed that mercury dissolution is directly proportional to cyanide concentration. At 10 g/ton of cyanide, approximately 42% of mercury was leached, whereas all gold was solubilized. During this study, miners recognized the risk associated with the cyanidation of mercury-rich tailings, and were aware of how much mercury is discharged to local streams and to the atmosphere. The active participation of miners in this study has led to the strengthening of their knowledge and awareness of mercury contamination, and has enhanced their understanding of the nature of the problem, as well as the weaknesses and strengths of the system they operate.     

Optimization of conventional Fenton and ultraviolet-assisted oxidation processes for the treatment of reverse osmosis retentate from a paper mill

According to current environmental legislation concerned with water scarcity, paper industry is being forced to adopt a zero liquid effluent policy. In consequence, reverse osmosis (RO) systems are being assessed as the final step of effluent treatment trains aiming to recover final wastewater and reuse it as process water. One of the most important drawbacks of these treatments is the production of a retentated stream, which is usually highly loaded with biorecalcitrant organic matter and inorganics; and this effluent must meet current legislation stringent constraints before being ultimately disposed. The treatment of biorefractory RO retentate from a paper mill by several promising advanced oxidation processes (AOPs) - conventional Fenton, photo-Fenton and photocatalysis - was optimized considering the effect and interaction of reaction parameters; particularly using response surface methodology (RSM) when appropriate (Fenton processes). The economical cost of these treatments was also comparatively assessed. Photo-Fenton process was able to totally remove the COD of the retentate, and resulted even operatively cheaper at high COD removal levels than conventional Fenton, which achieved an 80% reduction of the COD at best. In addition, although these optimal results were produced at pH=2.8, it was also tested that Fenton processes are able to achieve good COD reduction efficiencies (>60%) without adjusting the initial pH value, provided the natural pH of this wastewater was close to neutral. Finally, although TiO(2)-photocatalysis showed the least efficient and most expensive figures, it improved the biodegradability of the retentate, so its combination with a final biological step almost achieved the total removal of the COD.     

Sustainable sanitary landfills for neglected small cities in developing countries: The semi-mechanized trench method from Villanueva,Honduras

Open dumping is the most common practice for the disposal of urban solid wastes in the least developed regions of Africa, Asia and Latin America. Sanitary landfill design and operation has traditionally focused on large cities, but cities with fewer than 50,000 in population can comprise from 6% to 45% of a given country’s total population. These thousands of small cities cannot afford to operate a sanitary landfill in the way it is proposed for large cities, where heavy equipment is used to spread and compact the waste in daily cells, and then to excavate, transport and apply daily cover, and leachate is managed with collection and treatment systems. This paper presents an alternative approach for small cities, known as the semi-mechanized trench method, which was developed in Villanueva, Honduras. In the semi-mechanized trench method a hydraulic excavator is used for 1–3 days to dig a trench that will last at least a month before it is filled with waste. Trucks can easily unload their wastes into the trench, and the wastes compact naturally due to semi-aerobic biodegradation, after which the trenches are refilled and covered. The exposed surface area is minimal since only the top surface of the wastes is exposed, the remainder being covered by the sides and bottom of the trench. The surplus material from trench excavation can be valorized for use as engineering fill onsite or off. The landfill in Villanueva has operated for 15 years, using a total land area of approximately 11 ha for a population that grew from 23,000 to 48,000, with a land requirement of 0.2 m²/person year, a cover to waste ratio of 0.2, and an estimated soil surplus of 298,000 m³ that is valorized and used onsite. The landfill has been operated solely by the municipality with an operational cost in 2010 estimated at US$4.60 per ton. A modified water balance analysis at Villanueva shows negligible leachate generation from covered trenches and 700 m³/yr (60 m³/ha yr) from the two open trenches required for daily operation. If the site were an open dump, however, leachate generation is estimated to be 3900 m³/ha yr and contaminated runoff 5000 m³/ha yr. A simple model used to estimate dilution of generated leachate based on groundwater flow data and aquifer stratigraphy suggests that the leachate will be diluted by a factor of 0.01 in the aquifer. Leachate contaminants will not accumulate because the aquifer discharges to the Ulua River 2 km south of the landfill. While not suitable for all sites, the Villanueva method nevertheless serves as an excellent example of how a small city landfill with natural compaction of waste and attenuation of leachate can be sustainably operated.     

Vitamin B12 als biologisch aktive Modellsubstanz

A critical review on corrinoid biochemistry and physiology is presented. This includes: chemical synthesis of biologically important organocorrinoids and the correlation between their structures and coenzymatic activity; forms, distribution and transport of physiologically active corrinoids; methylcobalamin- and adenosylcobalamin-dependent enzymatic reactions and their physiological functions; and steric course of the adenosylcobalamin-dependent enzymatic rearrangements. Special attention is paid to the mechanisms of adenosylcobalamin-dependent enzymatic reactions.