Arsenic and fluoride removal from water using bone char

Arsenic and fluoride removal from water using bone char

Dissolved arsenic and fluorine species co-occurrence in groundwater is of great global concern due to the consequences arising from daily exposure. Millions of people around the world are experiencing health issues as a result of water contamination, especially in developing countries. Therefore, exploring a cost-effective and environmentally friendly adsorbent using waste material has gained great interest in recent years. The use of animal waste from a growing meat industry, provides a sustainable energy source and environmental benefits by reducing the amount of waste material to be disposed.

This study developed bone char and its composites using bone waste as the precursor. The initial objective was to study the effect of pyrolysis temperature (500, 650, 800 and 900 °C), variant residence time and purging gas on the characteristics of the bone char samples. The characteristics of these samples were compared using a range of analytical methods, which were used to interpret the mechanism of removal of fluoride (F-), arsenite [As(III)] and arsenate [As(V)] from water. The samples prepared at 900 °C had the highest removal capacity for As(III) and As(V) at 4.21 and 56.79 μg/g, respectively. On the other hand, a pyrolysis temperature of 650 °C was reported as the optimum temperature for the removal of F- from water.

Using four different isotherm models (Langmuir, Freundlich, Redlich-Peterson and Sips), the experimental data relating to As(III) and As(V) removal from water under optimal conditions was found to follow the Sips and Langmuir model, respectively. The kinetic models were examined at 0.5, 2.5, 5 and 10 mg/L of initial concentration for both As(III) and As(V) using kinetic and diffusion models. It was found that the main controlling rate of the adsorption process was intraparticle diffusion, while pore diffusion’s contribution in the adsorption process was limited to the lower concentrations of 0.5 and 2.5 mg/L. The outcome of the study showed that the removal of As(III) and As(V) from water was complex. Bone char was found to have a higher removal capacity of As(III) at higher concentrations (>20 mg/L). Thus, modifications are required to either increase the As(III) uptake by the bone char samples or oxidize them to As(V) to provide higher removal capacities.

The oxidation of As(III) to As(V) was examined by producing a composite through the coating of bone char (BC) with nanoscale titanium dioxide (nTiO2) as a photocatalyst. Two preparation methods were adopted for the coating process: after the pyrolysis process at 300 °C and during the pyrolysis process at 900 °C. The latter resulted in a phase change of the nTiO2 from anatase to rutile and showed higher As removal (57.3 % vs 24.8%) and oxidation efficiency (3.5 times more arsenate was produced) for both species compared to the composite prepared after the pyrolysis process. It was also found to have a higher removal capacity for both As species compared to unmodified bone char. The oxidation process was examined through the examination of three different levels of UV light (4, 8, and 12 W) during the experiment. By applying 12W of UV, a removal capacity of 281.36 μg/g was achieved for the composite, compared to the removal capacity of 195.76 μg/g using the uncoated BC900.

For the individual and simultaneous removals of As(III), As(V) and F-, fixed bed columns packed with bone char were used to quantify influent flow rate and concentration on breakthrough time during the removal of these contaminants. Breakthrough time was found to decrease with the increase in flow rate and influent concentrations. Four models were used to fit the experimental data relating to the characteristic parameters of the columns: the Yoon-Nelson, Thomas, and Bohart and Adams models. The best fit was provided by the Yoon-Nelson and Thomas models. The presence of coexisting anions was examined and it was found that they resulted in a decreased As removal from water, with F- posing the most significant effect. The improvement in As removal from water was examined by applying capitative deionization (CDI) technology to the columns. Promising results were achieved as there was a significant increase in breakthrough time using this combination.

This study confirms that valuable bone char and composites can be prepared from animal bio-waste to be utilized as adsorbents for various environmental applications. The structural and surface properties of bone char can be optimized by changing pyrolysis conditions to maximize the sorption capacity of the targeted contaminants.