Removal of Some Heavy Metals from Incinerator Bottom Ash Using Coconut ( Cocos Nucifera ) Husk

There has been a great limitation to the use of incinerator bottom ash (IBA) residue in the construction and manufacturing industries due to the high contents of heavy metals, which poses great threat to the environment and human health. This study was carried out to evaluate the efficiency of heavy metals (Fe, Zn, Cd and Pb) removal from domestic IBA filtrate using a biological material coconut (Cocos nucifera) husk as adsorbent and to remove the metals from the ash residue using the toxicity characteristic leaching procedure (TCLP). The mean percentage removal of the heavy metals for the incinerator bottom ash (IBA) filtrate varied from 75.84 99.42% for the two ash types. Similarly, the percentage removal of metals from the ash residues was between 92.34 and 99.97%. The data from this study showed that after the treatment, the heavy metals in the ash filtrate and residue were significantly reduced. Thus, it would be feasible to use the IBA residue as a partial replacement of cement in the construction and manufacturing industries for making monolithic blocks, interlocking bricks, concretes and road construction materials. This would help improve and provide good and effective waste management practice that would reduce the huge volume of ash residue from incineration combustion processes.


Introduction
Incineration is one of the major techniques for managing solid wastes in some developed and developing nations since it reduces the volume of waste by 80 to 90%.Municipal solid waste (MSW) consists of everyday items we use and dispose such as product packaging, furniture, clothing, bottles, food scraps, newspapers, appliances, paint, batteries etc.These wastes which come from homes, schools, hospitals, and businesses could cause detrimental effects on environmental media if not properly managed or disposed (EPA, 1980;El-Haggar, 2007).Depending on the municipality, solid waste could be managed using the four R's of waste management namely reduce, reuse, recycle and recover, however, incineration could be used for the final disposal of various types of waste.As waste generation rate increases, incineration processing costs increases, and available landfill space decreases, thus, the four R`s have become a central tenet in sustainable waste management efforts (Seadon, 2006;Davis, 2008;Suttibak & Nitivattananon, 2008;Tudor et al., 2011).Some techniques commonly used for the treatment or management of waste in addition to incineration include: landfill/ engineered landfill, compositing, mechanical biological treatment (MBT), pyrolysis and gasification, detoxifying hazardous waste, thermal desorption unit (TDU), etc.
Generally, solid waste incineration produces two main types of ash, which can be grouped as incinerator bottom ash (IBA), which are solid waste that are not completely burned on the grate and incinerator fly ash (IFA), which are solid and condensable particulate matter which leaves the furnace suspended in the combustion gases and are subsequently collected in emission control devices.The waste (ash) generated from incineration, usually ends up in two ways; disposal in landfill or reuse as secondary raw materials.In most developed countries where land is scarce and environmental controls are enforced, environmental policies tend to reduce landfill disposals as much as possible (Lam et al., 2010).
The ash residues produced from the burning of wastes could be used commercially as raw material in cosmetics

Ash Sampling
The incinerator bottom ash (IBA) was collected from the waste management unit of two oil Exploration and Production (E&P) companies.Twenty four (24) samples were randomly collected in triplicate and stored for analysis.The waste types used in this study include: domestic waste ash (DW) -obtained through the incineration of domestic waste other than hazardous waste and hazardous waste ash (HW) -obtained through the incineration of medical waste, other biological and hazardous waste.These ash wastes would subsequently be referred to as ash A for domestic waste (DW) and ash B for hazardous waste (HW).

Bio-adsorbent Sampling and Treatment
Coconut (Cocos nucifera) husk was collected and sun dried for approximately seven (7) days and ground into fine particles and sieved to obtain 120µm (micrometer mesh).The particles were treated with 0.1 M HCl and re-introduced into an oven at a temperature of 30 o C for 30 minutes and then preserved for further use.

Extraction and Determination of Metals in Ash Residue Using TCLP
Five (5) g of the crushed ash sample was accurately weighed into a sample extraction bottle and 96.5 mL of distilled water was added.This was covered with a watch glass and stirred vigorously for 5 minutes and the pH and electrical conductivity was determined.From the guidelines of EPA -SW-846 #1311 and DPR (EPA, 1992;DPR, 2011), extraction solution #2 was used for the metal extraction in the ash sample because the pH of the solution was greater than 5.If otherwise (i.e.< 5), extraction fluid #1 would have been used.Extraction fluid #1 was prepared by accurately measuring 5.7 mL glacial acetic acid, to which 500 mL of distilled water and 64.3 mL of 1 M NaOH was added and diluted to a volume of 1 litre.Extraction fluid #2 was prepared by accurately measuring 5.7 mL glacial acetic acid and added to distilled water make up to a volume of 1 litre.
For the metal extraction, 200 mL of extracting solution was added to 10 g of the crushed ash residue sample.This solution was shaken first for 2 minutes by hand to ensure saturation of the solid with the solution.Then the flask was shaken with an oscillator for 16 hours to ensure full saturation of the solid with the solution.The resultant slurry was centrifuged at 2000 rpm and the supernatant was carefully decanted through a glass funnel and the filtrate stored.This process was repeated three (3) times and the filtrate stored for the adsorption procedure while the concentration of heavy metals in the residues was determined using atomic absorption spectrophotometer.

Elution and Determination of Metals in Ash Filtrate
Ten (10) g powder of the pre-treated biological material -coconut (Cocos nucifera) husk was packed in a column stoppered at the tip with glass wool and 50 mL of the washed ash filtrate from the extraction of the residue introduced, allowed to drain through the packed column and repeated three times.The time taken for elution was recorded with a stop watch.Fifty (50) mL of the resultant eluate was digested using 5 mL concentrated nitric acid (AR).This reduced to the lowest volume possible (15 to 20 mL).Filtration was done after digestion to remove any insoluble material.The filtrate was then diluted to volume with distilled water in a 50 mL volumetric flask (APHA, 2005).The concentration of the heavy metals was analyzed using Atomic Absorption Spectrophotometer (AAS) by direct aspiration into a standardized computer interfaced with the instrument.

Results
The results obtained from the heavy metals analysis carried out on incinerator bottom ash are presented in Tables 1 and  2 with further illustrations in Figure 2-8.The tables indicate the concentration of heavy metals in the incinerator bottom ash filtrate before and after treatment with coconut husk as well as the extraction of metals from the ash residue.The washing of the ash resulted in filtrate, which was bio-treated using coconut husk and resulted in a decrease in the metal concentrations (Table 1).There was significant difference at levels of P<0.05 between concentration obtained before and after adsorption (Figure 2-3).Although ash B contained slightly a higher magnitude of metals than ash A, this could be as a result of the constituent of the hazardous waste, which comprises mainly of medical waste (syringes, needles, blades, forceps, and other related bio-medical waste) which was not destroyed in the incinerated residues.

Efficiency
The TCLP tes of metals and indicator for e inorganic, org influence of m Singh and Gad The The percentage removal of heavy metal was due to the greater availability of the exchangeable sites and surface area of the adsorbent.The coconut husk was adequately treated to remove traces of heavy metals that would interfere or cause contamination of the process.The removal of metal ion from the ash filtrates was effective since the bio-material was able to release an appreciate level of the metals present in the solution.The percentage heavy metal removal obtained from treating the two ash samples with the biological material (coconut husk), indicated that it is a safe and cheap material for the treatment of the incinerator bottom ash filtrates / solutions (Abdel-Ghani & El-Chaghaby, 2009).
The use of the cellulosic biological waste material as an adsorbent in treatment of incinerator bottom ash filtrate and the TCLP method of metal removal from solid ash waste would go a long way in solving environmental pollution problems that seems to be intractable to the society.The results and findings of this study are in agreement with previous studies made on heavy metal removal using coconut husk as an adsorbent (Agbozu & Emoruwa, 2014).

Conclusion
We conclude that cellulosic biological waste (e.g.coconut husk) was a good adsorbent for the removal of heavy metals from incinerator bottom ash filtrate and solution.Similarly, the TCLP method was effective in the removal of some major heavy metal contaminants in the incinerator bottom ash (IBA) residues, thus, enhancing the chances of using it in construction industries for making monolithic blocks, interlocking bricks, concretes and other road construction materials.Thus, effective removal of the contaminants from the ash residue could also mean an environmentally safe and convenient use of the raw material in cosmetics production including soap making, road construction, as foundation material, in noise barriers, as capping layer on landfill sites and in some countries as an aggregate in asphalt etc.

Table 1 .
Mean percentage metal removal from ash filtrate