KINETICS OF LEAD ADSORPTION BY ACTIVATED RICE HUSK ASH

ABSTRACT

The need to clean-up heavy metals that contaminate water cannot be over emphasized. This paper examined the kinetics of lead adsorption using activated rice husk ash. The influences of contact time, adsorbent dose, pH and temperature on adsorption were investigated. From the results obtained, by  analyzing the  effect  of  contact time,  it  was discovered that  maximum adsorption of about 90 % removal efficiency took  240 minutes using 0.2g of adsorbent. While in testing for the effect of adsorbent dose, the study found that for 180 minutesusing a dose of 0.4g of adsorbent, 90 % removal efficiency was obtained at pH 3.0.It was also discovered that the amount of lead ion adsorbed per gram of the adsorbent increased with decreasing concentration of lead (Pb) and that the percentage efficiency of adsorption increased with decreasing temperature. The two theoretical adsorption isotherms, namely, Langmuir and Freundlich were used to describe the experimental results. The Langmuir adsorption isotherm best fittted the model for the study with its correlation coefficient R2of 0.982. The adsorption of lead (Pb) followed the first order kinetics with the correlation R2  of 0.985 and was found to be pH dependent with a maximum value at pH 3.0. Based on the findings, the study therefore recommended that Industries, governments and individual households should consider the use of rice husk ash in wastewater treatment due to its high efficiency and relative cost effectiveness.

CHAPTER ONE

INTRODUCTION

1.1      Background of the Study

Lead-based compounds have been a major source of environmental contamination during these past decades. Lead has been reported to affect human health and as a possible cause of human cancer (Lin et al., 1996). Due to developments in technology, environmental pollution has become one of the most important contemporary problems. Industrial effluents have a great potential to cause lakes, streams, rivers and sea pollution. The ever increasing industrialization has increased discharge of heavy metals into the environment.   Heavy metals contamination exists in aqueous waste streams of many industries, such as metal finishing, electroplating, metallurgical work, mining,  chemical manufacturing, pesticides, fertilizers, dyes,  pigments, tanning and battery manufacturing industries (Kang et al.; 2007; Lesmana et al.; 2009). Heavy metals have been reported as priority pollutants, due to their mobility in natural water ecosystems. Their presence have caused severe environmental problems due to their toxicity even at low concentrations and insusceptibility to the environment. These heavy metals are non- biodegradable and tend to accumulate in living organisms, causing various diseases and disorders. For instance, cadmium exposes human health to severe risks, as it can aggravate cancer, bone damage, vomiting, diarrhea, kidney damage, mucous membrane destruction, and affect the production of progesterone and testosterone (Godt et al., 2006). So far, a number of efficient methods have been suggested by researchers for the removal of heavy metals among which are chemical precipitation, ion exchange, reverse osmosis, electrodialysis, ultrafiltration, nanofiltration, coagulation, flocculation, floatation, etc. However, these methods have several disadvantages such as high reagent requirement, unpredictable metal ion removal, generation of

toxic sludge etc. Adsorption process, being very simple, economical, effective and versatile has become the most preferred method for removal of toxic contaminants from wastewater. Table

1.1 shows the comparism using adsorption process with other conventional method.

Table 1.1: Comparison among wastewater treatment technologies. [Luqman

Chuah Abdullah et al., (2010)].

Physical and/or chemical process	Advantages	Disadvantages
Oxidation	Rapid process for dye removal.	High energy costs and formation by products.
Ion-exchange	Good removal of a wide range of metals and dyes.	Absorbent requires regeneration or disposal.
Membrane filtration technologies	Good removes of heavy metals and dyes.	Concentrated sludge production, Expensive
Coagulation/flocculation	Economically feasible	High sludge production and formation of large particles
Electrochemical treatment	Rapid process and effective for certain metal ions	High energy costs and formation of by-products
Ozonation	Applied in gaseous state:alteration of volume	Short half life
Photochemical	No sludge production	Formation of byproducts
Irradiation	Effective at lab scale	Required a lot of dissolved O2
Biological treatment	Feasible in removing some metals and dyes	Technology yet to be established and commercialized
Adsorption	(i) The high cost of AC limits its use in adsorption (ii)  Many varieties  of  low-cost adsorbents have been developed and   tested   to   remove   heavy metal ions (iii) Biosorption is a relatively new process that has proven very promising for the removal of heavy metal from wastewater	(i)  Removal  of  heavy metals from low wastewater concentration (ii) Adsorption efficiency depends on the type of adsorbents

On the other hand, lead (Pb) has been as one of the three most toxic heavy metals that have dormant long-term negative impacts on health, causing hepatitis, anemia, nephritic syndrome, brain damage, mental deficiency, anorexia, vomiting, malaise and encephalopathy (Deng et al.;

2006). Also, zinc poisoning can cause to nausea, vomiting, loss of appetite, abdominal cramps, diarrhea, headaches, anemia, damage to the pancreas, and decrease in levels of high-density lipoprotein (HDL) cholesterol. Conventional methods for heavy metal removal from water include ion exchange, reduction, precipitation, evaporation, electrochemical treatment, membrane filtration, reverse osmosis, electrodialysis  and carbon adsorption. Most of these methods may be expensive or ineffective when metals are dissolved at relatively low concentrations (Volesky, 1990).

Adsorption has been proved to be an excellent method to treat contaminated water, offering significant advantages like low-cost, greater availability, profitability, easiness of application and effectiveness in reducing the concentration of heavy metal ions to very low levels (Demirbas, 2008). Thus, Cost considerations can make it  expedient to  use local materials, produced in agricultural or industrial operations as adsorbents for toxics. Adsorption process is recommended for the  removal of low concentrations of metal ions in water.  This process implies the presence of an “adsorbent” solid that binds molecules by physical attractive forces, ion exchange, and chemical binding like activated carbon. Activated carbon is the most popular and widely used adsorbent for heavy metal removal in water treatment applications throughout the world. Though it is prolific in use, it remained an expensive material because its cost increased with it quality (Babel and Kurniawan, 2003).

The table 1.2 shows the maximum level of contaminants that drinking water should not exceed as recommended by the World Health Organization (WHO) and the National Agency for Food and Drugs Administration and Control (NAFDAC).

Table 1.2         Guideline for drinking water by the World Health Organization (WHO,

1998) and National Agency for Food and Drugs Administration and Control (NAFDAC), Nigeria.

No	Heavy metal	Max.    acceptable    conc.   (WHO,1998)	Max.     acceptable     conc.   (NAFDAC, 1998)
1	Zinc	5mg/l	5mg/l
2	Arsenic	0.01mg/l	0.0mg/l
3	Magnesium	50mg/l	30mg/l
4	Calcium	50mg/l	50mg/l
5	Cadmium	0.003	0.0mg/l
6	Lead	0.01mg/l	0.0mg/l
7	Silver	0.001mg/l	0.0mg/l
8	Mercury	0.001	0.0mg/l

Another type of adsorbent  is agricultural waste materials.  Agricultural waste materials are usually generated in large quantities. Rice husk, a type of agricultural waste material, which is a major by-product of the rice milling industry and is one of the most commonly available

lignocellulosic materials. For example, the annual rice husk produced in India is approximately

120 million tons.  Typically it is chemically composed of 20% ash, 38% cellulose, 22% lignin,

18% pentose, and 2% other organic components (James and Rao, 1986). Lignin, one of its major  components,  is  a  natural  amorphous cross-linked  resin that  has  an  aromatic  three- dimensional polymer structure containing a  number of functional groups such as phenolic, hydroxyl, car boxyl, benzyl alcohol, methoxyl, and aldehyde groups (Sarkanen and Ludwig,

1971), making it potentially useful as an adsorbent material for the removal of heavy metals from water.

1.2      RESEARCH PROBLEM

Heavy metals pose a risk of contaminating groundwater and surface water sources due to geogenic activities, industrial and agricultural sources (Daping et al.; 2015). The removal of low concentrations of heavy metals e.g., cadmium, lead and zinc from drinking, industrial and irrigation water is a recurring challenge, especially in developing countries. The removal of this heavy metals from our water and wastewater using adsorption process was proved to be an excellent way to treat contaminated water, offering significant advantages like low-cost, greater availability, profitability, easiness of operation and effectiveness in reducing the concentration of heavy metal ions to very low levels (Demirbas, 2008). Thus, cost considerations, greater availability and effectiveness has made it expedient to consider the use of local agricultural materials like rice husks to treat contaminated water.

I.3       RESEARCH OBJECTIVE

i.            To characterize rice husk ash;

ii.         To investigate the effect of pH on the adsorption of lead by rice husk ash(RHA);

iii.          To  investigate the  effect  of  temperature  on  the  adsorption  of  lead    by  rice  husk ash(RHA);

iv.          To determine the effect of contact time on the adsorption of lead by rice husk ash(RHA);

and

v.            To obtain the appropriate adsorption isotherm and kinetics for lead adsorption by rice husk ash(RHA)

1.4      SCOPE AND LIMITATIONS

The scope of this research is limited to determining the adsorption properties of rice husk ash. Several constraints were encountered during the course of the project. The constraints included inadequate laboratory facilities, finance and poor energy supply.

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