Abstract
Humic acid (HA) produced from brown coal, a relatively abundant and inexpensive material is currently being investigated as an adsorbent to remove toxic metals from aqueous solution. The influence of five parameters (contact time, solution pH, initial metal concentration, temperature and amount of adsorbent) on the removal at 20±1°C was studied. HAs were prepared from lignites by using alkaline extraction, sedimentation and acidic precipitation. Adsorption equilibrium was achieved in about 60min for Cr3+ ion. The Langmuir adsorption isotherm was used to describe observed sorption phenomena. The maximum adsorption capacity of 0.17mmol for Ilgın (HA1), 0.29mmol for Beysehir (HA2) and 0.18mmol Ermenek (HA3) and 0.17mmol of Cr3+/g for activated carbon (AC) was achieved, respectively at pH of 4.1. More than 84% of Cr3+ was removed by HA2, 54% by HA3 and 51% by HA1 and 50% by AC from aqueous solution. The adsorption was strongly dependent on pH but independent of ionic strength and metal ions. The adsorption of Cr3+ was higher between pH 4.1 and 5.1 for all HAs and maximum sorption was observed at pH 4.1. The rise in temperature caused a slight decrease in the value of the equilibrium constant (Kc) for the sorption of Cr3+ ion. Complex mechanisms including ion exchange, complexation and adsorption and size exclusion are possible for sorption of Cr3+ ion on HAs.
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