Effects Of PH Environment And Additives On Solidification/Stabilization And Potential Reuse Of MSW Incineration Fly Ash

Fly ash(FA),a product of municipal solid waste incineration(MSWI),has been classified as a kind of hazardous waste due to its high content of heavy metals.Fly ash may be reused or disposed of at landfill sites,and thus poses threats to both the environment and human health.This study sought to establish scientific bases for accurate selection of suitable pH for storage,pre-treatment conditions,and reuse of MSWI FA.As a result,the study evaluated:(1)the potential of MSWI FA sample from Xinghuo waste incineration power plant in Wuhan to solidify/stabilize the heavy metal(Cu,Pb,Zn,Cr,Cd,As and Mn)contents,when leached under different pH conditions,(2)the possible effects of washing pre-treatment on chloride and heavy metals in MSWI FA,(3)the solidification/stabilization and potential reuse of MSWI FA through cement addition,(4)the S/S and potential reuse of MSWI FA through the addition of cement and sea sand,and(5)the S/S and potential reuse of MSWI FA through the geopolymer method.The concentration of heavy metals in the leachates either during the pH dependent leaching or through the use of the United States Environmental Protection Agency(USEPA)toxicity characteristic leaching procedure(TCLP)were assumed to inversely reflect the extent of S/S in all the studies.The study findings showed that the untreated MSWI FA in the pH dependent leaching contained higher levels of heavy metals,which were above the acceptable limits.Extremely acidic conditions favoured heavy metal leaching compared to extremely alkaline conditions.The extent of Solidification/Stabilization(S/S)of heavy metals was generally very low under highly acidic conditions(pH ? 4),but increased with increasing pH.All the metals solidified/stabilized in pH media of 5 to 11,except Zn which was detected in the entire pH range.It can be concluded from the study on the effects of pH dynamics on S/S of MSWI FA that,changing landfill conditions which can affect the pH environment,will increase heavy metal leaching when the pH ? 4.As a result,waste which was initially classified as non-hazardous may later pose harmful risks to both humans and the environment alike.A pH range of 5 to 11 was therefore proposed as the optimum for the treatment,reuse,and disposal of the ash sample.This section of the abstract is on the second part of the research that deals with S/S and potential reuse of MSWI FA.Even though MSWI FA has been classified as a category of hazardous waste due to its high amount of heavy metals and chloride,it can either be reused in other sectors or disposed of at landfill sites,and therefore poses environmental and public health risks.This study sought to establish the effects of water washing pre-treatment on S/S and the quality of MSWI FA for potential reuse and/or disposal.As a result,it evaluated the effects of water washing pre-treatment on MSWI FA sample from the Xinghuo waste incineration power plant in Wuhan,P.R.China.Thereafter,it was used together with cement,cement and sea sand and through the geopolymer method to study the S/S of heavy metals(Cu,Pb,Zn,Cr,Cd,As,and Mn)and its potential reuse.Initial analysis of the FA sample proves that it did not meet the USEPA-1311 TCLP regulatory standard of 2011,as a result,a two stage water washing pre-treatment was carried out on it in an attempt to improve its quality and increase its quantity in cementitious material.After the washing pre-treatment,the FA sample met the USEPA-1311 regulatory standard,thereby making it safe for reuse and/or disposal.Furthermore,leachate from the washing pre-treatment stage also met the regulatory standards of China(GB 8978-1996)and the USEPA-1311.More than 98 % of each heavy metal was successfully removed/immobilized by the washing process.The washing process was excellent at S/S of cadmium(99.99 %)and very good at the rest of the heavy metals.About 98.12 % of Ni,98.61 % of Cr,99.95 % of Zn,and 99.96 % of Co and Cu were effectively removed/immobilized.This part deals with S/S and potential reuse of MSWI FA through cement addition.The compressive strength of the samples prepared increased as the quantity of pre-treated FA decreased after 28 days of curing.Also,all the heavy metals in the samples prepared from the pre-treated FA met the 2011 USEPA-1311 TCLP regulatory standard for reuse and safe disposal.The heavy metal leachability and compressive strength tests showed that,the effect of cement-based S/S was influenced by its percentage addition.The highest compressive strength was attained when 45 % of the pre-treated FA(FAC55)was replaced by cement.However,FA applications of up 70 % yielded good results which passed the 1989 USEPA recommended compressive strength value of 0.34 MPa(50 psi)and the 2011 USEPA-1311 TCLP standard.The second section of the S/S studies is discussed in this part,which has also been published in an SCI indexed journal.It deals with S/S and potential reuse of the pre-treated MSWI FA through cement and sea sand addition.The concentration of heavy metals,density and compressive strength of all the samples prepared with pre-treated FA increased as its content decreased after 28 days of curing under room conditions.All the samples prepared met the 2011 USEPA-1311 TCLP regulatory standard.Chromium(Cr)was not detected in all the samples tested,thereby making this an excellent method for its immobilization.Also,there was a stronger correlation between density and compressive strength of the samples.The compressive strength of all the samples passed the 1989 USEPA recommended value of 0.34 MPa(50 psi)after 28 days of curing,thereby making this method suitable for potential large scale application of pretreated MSWI FA as cement supplement.The fifth part of this abstract is on S/S and potential reuse of MSWI FA through geopolymerization.This study confirmed that water washing is an effective way of reducing chloride and heavy metals in MSWI FA,and for increasing its quantity in cementitious mixtures.It applied only pre-treated MSWI FA in a geopolymer process which yielded a compressive strength of 4.67 MPa after 28 days of curing under ambient conditions,whilst a sample made up of 75 % FA produced the highest compressive strength(4.68 MPa)under the same conditions.Nevertheless,all the samples yielded high compressive strength after 28 days of curing which passed the 1989 USEPA value of 0.34 MPA(50 psi)and the 2011 USEPA-1311 TCLP standard.Also,the sample in which only FA was used possessed superior S/S ability of almost all the heavy metals understudied,when compared with the rest of the samples.Analysis of the physical properties of the samples revealed that,as the content of FA increased,density decreased,and compressive strength increased.Which suggest that the amount of metakaolin used actually had a retarding effect on the compressive strength.Moreover,the amount of FA generally increased with the potential heavy metals S/S in the samples.Relationship between the FA content,density and compressive strength of the samples was very strong.Compared with samples prepared with only FA and cement,and FA,cement and sea sand,the compressive strength of all the geopolymer samples increased with the quantity of FA.