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Batch reactor design for algal biosorption of mercury-contaminated acidic water in the presence of competing ions

( Rosamond Tshumah-mutingwende ),( Fumitake Takahashi ),( Ewa Cukrowska ),( Julien Lusilao-makiese ) 한국폐기물자원순환학회(구 한국폐기물학회) 2017 한국폐기물자원순환학회 심포지움 Vol.2017 No.1
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The anthropogenic release of mercury, a known mutagenic and teratogenic element, is by far the largest threat to the well-being of ecosystems and human health. The rudimentary and unscrupulous operations of conventional miners and Artisanal and Small scale Gold Miners (ASGM) in Zimbabwe have led to an unprecedented increase in mercury exposure to humans through the contamination of potable water sources. Reports of elevated blood levels of mercury in human blood and hair and the general deterioration of affected rivers are evidence of a rising problem. One such river is Ngwabalozi River in Mzingwane District in Zimbabwe. An area of concern for Ngwabalozi River is the high mercury levels of 0.31 mg L-1 and low pH of 3. The cyanidation technique for gold recovery used at a nearby conventional mine, How Mine, results in the release of other heavy metals such as iron and copper from the gold ore. The discharge of mine waste water rich in heavy metals and sulphates; and the use of nitric acid by artisanal miners for the removal of impurities from the gold-mercuiy amalgam have contributed to the low pH of the water. This has made the water in Ngwabalozi river unsuitable for irrigation and domestic use. Hence, the purpose of this study is to design a 1000 L batch reactor for the adsorption of mercury from acidic waters. Due to the myriad of chemicals used by small scale gold miners and conventional mines, mercury abatement by algal adsorption must consider the presence of potentially competing metal cations. From the foregoing, adsorption kinetics in the presence of Fe2+, Cu2+, Zn2+, Co2+ cations were studied. Single component (Hg only) batch experiment adsorption studies displayed a rapid uptake of mercury. More than 99% of the Hg in solution was removed within the first five minutes of contact and equilibrium was attained after ten minutes. High adsorption capacities (up to 800 mg kg-1 at pH 3) were obtained at the optimum Hg concentration of mg 1-1. The experimental data fitted very well with the pseudo-second order kinetics model. In competitive adsorption studies, a maximum extraction efficiency of 67.2% was obtained after 60 mins of contact at pH 3 and initial metal cation (Hg2+, Fe2+, Cu2+, Zn2+, Co2+) concentration of 1.0 mg L-1. The selectivity of metal cations by Cladophom sp. was in the following order: Hg2+ > Fe2+> Cu2+> Zn2+ > Co2+. The effect of the presence of Cu2+, Zn2+ and Co2+was negligible. The adsorption data was modelled on the multicomponent Freundlich, Langmuir and Sips equilibrium isotherm models. Nonlinear regression and Solver were used to determine the best fit equation. Using the equilibrium mass balance equation, scaling up revealed that the mass of algae required to reduce Hg2+ in presence of Fe2+ to the WHO standard of 0.006mg L-1 ranged from 13.4 kg to 54.7 kg for a minimum and maximum initial Hg2+ and Fe2+ concentration of 0.25 mg L-1 and 1.0mg L-1 respectively; for a contact time of 60mins in a 1000 L batch reactor.

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