ADSORPTIVE CHARACTERISTICS OF PHOSPHORUS ON FOUR BIOCHAR TYPES

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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

  •   Background

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).