Abstract
Precipitation of metals as metal sulphides is a practical way to recover metals from mine water. Sulphide precipitation is useful since many metals are very sparingly soluble as sulphides. Precipitation is also pH dependent. This article investigates the precipitation of metals individually as sulphides and assesses which metals are precipitated as metal hydroxides by adjustment of the pH. The precipitation of different metals as sulphides was studied to determine the conditions under which the HS− solution from the sulphate reduction reaction could be used for precipitation. H2S gas and ionic HS− produced during anaerobic treatment could be recycled from the process to precipitate metals in acidic mine drainage (AMD) prior to anaerobic treatment (Biological sulphate reduction), thereby recovering several metals. Precipitation of metals with HS− was fast and produced fine precipitates. The pH of acid mine water is about 2–4, and it can be adjusted to pH 5.5 before sulphide precipitation, while the precipitation, on the other hand, requires a sulphide solution with pH at 8 and the sulphide in HS− form. This prevents H2S formation and mitigates the risk posed from the evaporation of toxic hydrogen sulphur gas. This is a lower increase than is required for hydroxide precipitation, in which pH is typically raised to approximately nine. After precipitation, metal concentrations ranged from 1 to 30 μg/L.
Highlights
Access to potable water is not universal
After adjusting the pH, precipitates after the hydroxide precipitation could be recovered, but this step could be avoided by collecting all the precipitates after the subsequent hydrogen sulphide precipitation
The NaOH precipitation was tested, and the results showed that not all may be precipitated as sulphides after raising the pH ≥ 8 before adding the HS- solution (Table 5)
Summary
Access to potable water is not universal. Only 3% of the Earth’s water is fresh water and, the contamination of existing water supplies is a problem of global significance. Industrial wastewater pollutes water supplies and metals are among the most common contaminants contained in this wastewater [1]. The presence of various metals, such as Cr, Pb, Zn, As, Cu, Ni, Co, Cd, and Hg, causes disturbances in circulatory, gastrointestinal, and nervous systems. They can affect various organs, including the central nervous system, leading to mental disorders, and damage the blood constituents, which may result in damage to the lungs, liver, kidneys and other vital organs, promoting several disease conditions [2] and possibly leading to blindness, deafness, brain damage, loss of fertility, cancer, and many other severe health problems that may be fatal [3]
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