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Magnetic-based materials a new innovation in waste water treatment

by Associate Professor Dr Mubarak Mujawar

Malaysia has abundant biomass waste resources, particularly municipal and agricultural waste. Currently, about 90 million metric tonnes of renewable biomass such as empty fruit bunches (EFB), kernel shells and tree trunks are produced from approximately three million hectares of oil palm plantations. Of the amount, EFB constitute 12.4 million tonnes and palm shell waste 2.4 million tonnes.

As in most tropical countries, agricultural by-products or biomass materials in Malaysia are abundantly available, cheap and, more importantly, renewable. The reuse of such solid wastes can be beneficial to regional economies because high-value products can be obtained from such low-cost materials.

However, these materials can also pose environmental problems such as the emission of hazardous and toxic chemicals like dioxins from the burning of biomass.

Such environmental concerns, as well as the limitations of landfill sites and the high cost of treating leachates from the sites, have prompted researchers to look into the effective and commercially valuable utilisation of the EFB.

Biochar is a low-cost, stable material produced from pyrolysis carbonaceous biomass. With its carbon storage capacity, it can help reduce greenhouse gases and the presence of carbon dioxide, methane and nitrous oxide in soil. It is also a soil enhancer that can increase cation-exchange capacity for enhanced soil fertility, soil acid moderation, high rate of water retention and high amount of soil microbes.

Activated carbon (AC) is one of the good adsorbents widely used in the industrial sector but regeneration and desperation problems occur in its industrial application, resulting in the difficulty of separating powdered biochar like powdered AC from the aqueous solution.

To overcome these shortcomings, an innovative technology known as magnetic biochar is being developed to remove pollutants like heavy metals, phosphate and organic compounds from aqueous solutions.

Water resources are also being polluted from various sources and the removal of toxic metals from water is one of the biggest challenges in water treatment.

There are many conventional methods being used to remove metal ions from water and wastewater such as oxidation and reduction, precipitation, membrane filtration, ion exchange and adsorption. Among these methods, the most effective process is adsorption because some of the used adsorbents can be regenerated using suitable desorption processes. Therefore, magnetic biochar is a promising solution for the adsorption of heavy metals and the reduction of environmental problems.

To remove the sorbents effectively, magnetic biochar is introduced to commercially available sorbents so that the magnetic separation technique can be applied to separate organic arsenic from magnetic iron oxide, selenium and phosphate.

Microwave technology is also being introduced to address the problem. Microwave irradiations have the capability of molecular level heating, which leads to homogeneous and quick thermal reactions. It also gives better control, energy efficiency and cost effectiveness over conventional heating. The thermal conversion of the bio-material requires a uniform heating to maintain the overall quality of the biochar.

The heating rate should be fast to reduce production costs and microwave heating can heat the bio-materials faster and uniformly throughout the bulk. The microwaves penetrate the material and the microwave energy is converted to heat energy. In this way, heat is generated throughout the bulk of the material.

Ultimately, microwave heating improves both the quality and processing time and magnetic biochar production via microwave heating to turn agricultural waste biomass into valuable products is highly feasible.

The production of magnetic biochar depends on various process parameters such as reaction power, impregnation ratio, reaction time, and inert gas flow rate, but the main advantage is that it can be directly produced using microwave heating through a single stage of activation compared to conventional methods which require multiple stages of heating. Furthermore, after the adsorption of the pollutants, the magnetic biochar can effectively be separated by applying the magnetic separation technique.

The loading of magnetic medium on to the biochar increases the cost of the sorbent but the increased cost can be compensated by the renewability of the sorbent. Thus, magnetic biochar is recognised as a highly efficient, cost effective and more environment-friendly sorbent for various types of pollutants and for heavy metal and dye removal. Furthermore, the magnetic properties of biochar can be used in other applications to replace AC due to its high surface area, high porosity and high adsorption capacity.

Through their studies, researchers have demonstrated a new dimension to the applications of AC and are designing treatment systems that have inherently desirable characteristics rather than continually needing to control the circumstances. A good example is magnetic biochar separated by magnetic field, so there is no need for additional energy or materials to separate purified drinking water from metal laden adsorbent.

Dr. Mubarak Mujawar is an Associate Professor in the Department of Chemical Engineering, Faculty of Engineering and Science, Curtin Malaysia. He can be contacted at +60 85 443833 or by e-mail to mubarak.mujawar@curtin.edu.my.