ABSTRACT
Biochar prepared from cocoa pod, sawdust, rice straw and husk wastes may provide new low cost technology for environmental management with emphasis on P removal from waste water to minimize eutrophication and to enhance P availability in tropical soils. To achieve this, the sorption characteristics of the biochar types would have to be understood. In this study, laboratory experiments were conducted to investigate the P adsorptive characteristics on four biochar types derived from cocoa pod, sawdust, rice husk and straw. Batch sorption experiment was conducted to investigate the time for maximum adsorption by shaking at two (2) hours intervals between 2 to 24 hours. X-ray diffractograms of the various biochars were obtained to help elucidate possible mechanisms of adsorption. Effect of pH on adsorption was also studied for initial pH ranging between 2 and 12 and initial P concentrations between 0.4 mM and 1.6 mM. Results from the research revealed that six (6) hours of shaking time was sufficient to achieve maximum adsorption onto the various biochars. Optimum pH for adsorption occurred at equilibrium pH of 5.7 for rice husk, 6.2 for sawdust, 6.7 for cocoa pod and 7.2 for the rice straw biochar. The isotherms indicated that the amount of P adsorbed increased with increasing equilibrium P concentrations. Increases in equilibrium pH above 7.2 led to decreases in adsorption for all the biochar types. Rice husk and sawdust biochar types were found to have the highest affinity for P with estimated maximum adsorption of 7300 mg/kg P. Phosphate adsorption mechanism varied with biochar type. Surface precipitation of P by Ca and Mg was proposed as an important mechanism of P adsorption on the sawdust biochar. Magnesium precipitation of P was also proposed as a mechanism of P removal by the rice husk biochar. Both electrostatic attraction and ligand exchange reactions by periclase (MgO) with P could be the main mechanisms of adsorption on cocoa pod, rice straw and sawdust
biochar types. Phosphorus adsorption via ligand exchange and or electrostatic attraction could have accounted for P removal by the rice husk biochar.
CHAPTER ONE
INTRODUCTION
There are many different types of biomass resources in Ghana including agricultural crop residues, agricultural by-products, forestry residues, wood waste, and organic portion of municipal solid waste (Duku et al., 2011). The major crop residues generated in the country include rice straw, rice husk, cocoa pods, stalk of maize, sorghum, millet and pineapple peels, etc. Ghana produces 13000 tonnes of waste daily; a bulk of which is organic, but the country lacks the appropriate infrastructure to manage the waste (Foray, 2012). Any technology that will transform these organic wastes into useful material will therefore be of immense benefit to the country.
There are mountains of sawdust in almost all districts of the country because of the activities of wood industries consisting of sawmills and carpentry workshops. These sawmills generate 20% of the total volume of wood milled as sawdust alone, excluding other wood waste (United Nations Industrial Development Organization [UNIDO], 2009). The mountains of sawdust are mainly disposed of through aerobic burning which culminates in the release of greenhouse gases (GHGs).
As a national policy to cut down on rice importation, there has been a conscious effort to boost rice production in Ghana. This has led to the generation of large volumes of rice straw and husk. Rice husk as an agricultural waste abounds in almost all rice-growing centres in Ghana and accounts for 23% of total paddy weight (Frimpong-Manso et al., 2011). The high C: N ratio of rice husk makes the material not easily decomposable and thus unsuitable for use as soil amendment. Consequently, the material piles up breeding rodents such as mice that in turn attract snakes to the breeding sites. Rice straw is also a waste generated in the rice growing fields after harvesting.
Some of the rice straw are being used as feed for cattle and are occasionally incorporated into the soil during ploughing. The main method of disposal of these two materials has however, been through aerobic burning which also culminate in the release of GHGs.
Ghana is presently the world’s second largest producer of cocoa beans. In the year 2012, Ghana produced over 1,000,000 metric tonnes of cocoa beans (USDA, 2012). However, the production of one tonne of marketable cocoa entails the harvest and breaking of approximately between 25,000 and 30,000 pods (Sustainable Tree Crop Production [STCP], 2007). These cocoa pods are dumped near farm steads on the plantations and become a significant source of disease inoculum when used as mulch on the plantations (Figueira, 1993). Some decades ago, the cocoa pods were ashed and used in the preparation of soap. However, with the influx of imported soap, the locally manufactured soap from cocoa pods is not so popular with Ghanaians leading to the piling up of the cocoa husk. Any technology that will transform the pods into a useful material will go a long way in reducing tonnage of waste produced from cocoa.
Pyrolysis can be a potential promising method of managing rice husk, rice straw, sawdust and cocoa pod compared to the current landfilling, incineration, or direct agricultural utilization, with their attendant secondary pollution problems (Lu et al., 2011; Hwang et al., 2007). The pyrolysis process tends to reduce the volume of bio-solids, eliminate pathogens and change the organic matter into bio-fuel, bio-oil and biochar (Lu et al., 2011; Domi’guez et al., 2006). Biochar pyrolysed from agricultural waste has large surface area and contain high elemental carbon and phosphorus, with a large amount of exchangeable cations. These positive attributes could be explored to improve soil fertility (Hossain et al., 2010, 2011; Lu et al., 2011).
