POLLUTION IMPACT, PHYTOREMEDIATION AND KINETICS OF ADSORPTION OF SELECTED METAL IONS FROM RIVER AND WASTE WATERS USING ARROWROOTS (AMARANTA ARUNDINACEA)

Abstract

Increased human activities have led to a rise in pollutants in the environment among them heavy metals. Conventional technologies for removal of metal ions from polluted water are numerous. Most of these methods are, however, expensive and sometimes inefficient. Adsorption utilizing natural adsorbents is found to be cost effective and adaptable to local needs. A study on the removal efficiency of Cu2+, Mn2+, Zn2+, and Fe2+ metal ions from River Malakisi water and wastewater from Eldoret municipal treatment plant using arrowroot biomass adsorbent has been done. The rhizomes of arrowroots planted in distilled water were harvested, sun and oven dried and then used for biosorption. Adjusting pH from 2.5 to 9.5, residual metal ion levels were measured using UV visible spectrophotometer. The procedure was repeated using residual metal ion levels at intervals of 10 minutes and at optimum pH of 5.5. Batch experiments were done by monitoring two model solutions of low 5 and high 25 ppm for mixed and single solutions containing the metal ions. A model constructed wetland was also set up by planting arrowroots in the influent and effluent of municipal wastewater where the % metal removal rates were compared with the wastewater that does not pass through the wetland. Other parameters determined in river water were coliforms, BOD, COD, anionic pollutants: NO3- , PO43-, SO42- and F- , total hardness, turbidity, conductivity and pH. The Langmuir and Freundlich isotherm models, kinetic order of reaction and inter-particle diffusion models were evaluated to study the adsorption process for the metal ions using arrowroots. The paired t-test statistical parameter and relative treatment efficiency index were also used to compare the percent metal removal efficiency from municipal wastewater with and without live arrowroot plants. The results showed that all the heavy metals, except Zn, coliforms and turbidity were above the WHO limits while other pollutants were within, implying the general pollution of River Malakisi. The % metal removal efficiency from river water by arrowroots followed the trend, Fe2+ > Cu2+ > Mn2+> Zn2+. The R2 for both Langmuir and Freundlich isotherms showed that the adsorption data for the metal ions in river water using arrowroots best fitted the Freundlich isotherm, that is mainly by hetero layer mechanism. Results of batch experiments showed that adsorption of all the metal ions in model solutions containing mixed ions of low and high concentrations, adopted pseudo second order mechanism while solutions containing single ions at high concentrations adopted pseudo first order mechanism. Adsorption of the metal ions by dry arrowroots from model solutions involving higher concentrations than river water, best fitted the Langmuir isotherm. The metal removal rates in wastewater cultivated with live arrowroots were however, higher than that without arrowroots, suggesting that arrowroots could be of significant benefit providing a tertiary wastewater treatment alternative.