Modifications of coconut pith as mercury adsorbents for industrial applications

Abstract

Contamination of wastewater by mercury ions poses a great concern due to its toxicity and threat to the public health and ecological systems. This study was conducted to investigate utilization of coconut pith (CP) as adsorbents for the removal of mercury ions, Hg(II) and MeHg(II). The CP underwent several modification processes: pre-treatment; silane-grafting and dye-loading, aiming for better Hg(II) and MeHg(II) adsorption performance. The adsorption performance study was conducted in batch and continuous adsorption system. The physical and chemical properties of CP adsorbents changed after modifications. The silanegrafting using mercaptopropyltriethoxysilane (CP-MPTES) and dye-loading using Reactive Red 120 (CP-RR) resulted in the highest removal efficiency towards both mercury ions. This is due to the presence of functional groups which have high affinity towards both mercury ions. Batch adsorption studies found that the adsorption capacity of both mercury ions was dependent on initial pH, adsorbent dosage, initial concentration, contact time and temperature. The maximum adsorption capacity of Hg(II) onto CP-Pure, CP-MPTES and CP-RR was 2.60, 2.61, and 2.60 mmol/g, while 0.50, 1.13 and 0.76 mmol/g was observed for MeHg(II), respectively. The equilibrium and kinetic data analyses found that the mechanism of mercury ions adsorption onto CP adsorbents is a combination of physical and chemical processes. The high regenerability was only observed in Hg(II) adsorption. The competence of Hg(II) and MeHg(II) adsorption in oilfield produced water and natural gas condensate samples, respectively, found that the presence of other metal ions reduced the adsorption performance of the mercury ions. The mercury ion adsorption in continuous fixed-bed adsorber studied at various conditions showed that the increase of flow rate and initial concentration caused the exhaustion time (texh) to occur earlier, but higher bed height prolonged the texh. The breakthrough curves of mercury ions adsorption were analyzed using Thomas, Bohart-Adam, Yoon-Nelson, Wolborska and Clark models. Thomas and Yoon-Nelson models fit most of the experimental data. However, empty bed contact time analysis found that the Yoon-Nelson model is more applicable to predict the breakthrough curves of the scale-up adsorber. The regenerability studies had low performance towards Hg(II) adsorption and fair performance towards MeHg(II) adsorption. In overall, the batch and continuous mercury ion adsorption results show the potential application of the CP adsorbents as low-cost adsorbent for industrial mercury ion removal process.