Water, Its Pollution and Abatement Technological Strategies: Application of Cheap and Environmentally Friendly Local Resources
Synopsis
Increased domestic, industrial and agricultural practices have led to a rise in wastewater discharges, hence water pollution. Public awareness of water pollution events together with the effluent discharge standards now set; have led producers to devise various ways of treating these effluents before discharge. Such ways should, however, be environmentally friendly and cost effective. One such approach to wastewater treatment is the application of natural products particularly algae, which have been found to be good adsorbents for certain contaminants. Biosorption of the heavy metal ions, Cd2+, Cu2+, Pb2+ and Zn2+ by the brown algae, ascophyllum nodosum and moringa oleifera has been studied by measuring the efficiency of their removal from ‘waters’ using contact time and pH as the variables in a batch-process series of experiments. Biosorption of mixtures of the metal ions to study their competitive effects within the algae at pH’s of 5 and 9 and moringa o. at pH 4 has also been done. The concentration of metal ions adsorbed were measured using inductively coupled plasma-optical emission spectrometer. Flow injection experiment was done by incorporating a column packed with the algae to determine its maximum capacity for metal biosorption. The effects of stirring on metal biosorption and particle size distribution of three pore size categories of the algae using the mastersizer equipment were also investigated. Dry moringa o. were de-shelled by hand and crushed by paste and mortar, washed and dried. Forty grams of powder were then shaken with 400 mL of acetone /ethanol mixture for 30 minutes, centrifuged and dried in the oven at 30 0C overnight. The results for algae show that for the single systems, Cd2+ were the most efficiently biosorbed metal ions while Pb2+ were the least; following the order Cd2+>> Zn2+ ≥Cu2+> Pb2+ and Cd2+>Zn2+>Pb2+>Cu2+ at pH’s 5 and 9, respectively for mixed ions for algae of pore size < 1 mm. Reduction in algae size lowered metal biosorption and also changed the order to Zn2+>Cd2+>Cu2+>Pb2+. A pH of 5 was found to be optimum for adsorption of these metal ions. The metal uptake increased with contact time based on a solid-to-liquid ratio (mass of algae to volume of water) of 1:100. The algal biomass demonstrated a Langmuir adsorption-like isotherm, with a steep rise in the initial slope of the kinetics prior to an equilibrium-like asymptote and stirring was found to lower the time to attain equilibrium. Maximum % biosorption could only be attained after 1 hour of contact. Modified algae facilitated by treatment using KMnO4 and Na2CO3 enhanced heavy metal biosorption with the former offering better properties. The untreated algae were, however, found to have a maximum capacity for metal biosorption of more than 4 hours. From particle size distribution, although the majority of algae particles in each category were of the largest size, their % volume occupied was low. For moringa o. single systems, the larger fraction (< 1 mm) offered better biosorption abilities than the smaller one (> 500 µm) for Cd and Zn. Generally, the smaller particle size fraction (< 500 µm) was least in biosorbing metals and the order of biosorption for all particle sizes was Zn > Cu > Cd > Pb. Although treatment improved moringa’s biosorption abilities, KMnO4 proved a better enhancer of metal biosorption than Na2CO3. At low pH, the trend of biosorption for moringa o. was Zn > Cu >Cd > Pb while at high pH Cd was the most with Pb the least and the order was Cd ≥ Zn > Cu >Pb. Both algae and moringa adsorption followed Langmuir model which proceeds through surface and mono layer mechanism than the Freundlich which is hetero based. It was also found that all metals except Zn with algae adsorbent adopted pseudo second order reaction.

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