Removal of some heavy metals onto mechanically activated fly ash: Modeling approach for optimization, isotherms, kinetics and thermodynamics

https://doi.org/10.1016/j.psep.2017.04.012Get rights and content

Highlights

  • The mechanical activation increases adsorption capacity of fly ash.

  • Equations can be derived using a limited number of experiments by RSM.

  • Raw and activated fly ashes fit the Langmuir isotherm very well.

  • Activated fly ash can be used as a low-cost, available and waste adsorbent.

Abstract

This study was recommended for the purpose of showing that mechanically activated fly ashes can be firstly used as an adsorbent for the removal of some heavy metals such as Cu(II), Mn(II), Ni(II), Pb(II) and Zn(II) from aqueous solutions. And with the idea that the adsorption capacity of fly ash can be modified by increasing the surface activity with mechanical activation, adsorption properties of activated fly ash samples were examined. Adsorption experiments were conducted on both fly ash samples. The optimizing of process parameters was evaluated employing response surface method (RSM) and adsorption mechanism can be explained by the quadratic model. The activated fly ash showed higher removal percentage (96% (w/w)) of all metals than the raw fly ash under the same experimental conditions. A pseudo-second order kinetic model fitted the adsorption data well. Moreover, the adsorptions of heavy metal ions on raw and activated fly ashes fit the Langmuir isotherm very well. Thermodynamic parameters (ΔG°, ΔH° and ΔS°) were showed that the adsorption process is suitable, spontaneous and endothermic. The adsorption/desorption behavior of a multi-metal mixture on raw and activated fly ashes was firstly investigated. The adsorption/desorption percentages of Cu(II) and Pb(II) ions were higher than other ions. Mechanical activation results in decrease in particle size, increase in surface roughness and agglomerated particles as evident by characterization using XRD and SEM techniques. The results indicated that the heavy metal pollutants of aqueous solutions could be reduced using activated fly ash as a low-cost, available and waste adsorbent.

Introduction

The term heavy metal is applied to a group of elements having atomic density value of more than 6 g/cm3. Heavy metals such as arsenic (As), copper (Cu), cadmium (Cd), chromium (Cr), nickel (Ni), zinc (Zn), lead (Pb), mercury (Hg) and manganese (Mn) are major pollutants of water reserves because of their toxic, non-biodegradable and persistent nature (Bilal et al., 2013). Increased industrialization and population have a negative impact on the environment through the discharge of waste containing heavy metals (Majumdar et al., 2010). Effluents from industries such as mining, textile, tannery, metal-plating, petro-chemical, battery and fertilizer production, contain metals that are deposited in the soil and seep into water courses affecting aquatic and animal life (Bilal et al., 2013). Thus, the toxicity, bio-accumulation and persistence of these metals are transmitted through the food chain and the environment to cause environmental and human health problems such as blood and brain disorders (especially in young children), nephropathy, colic-like abdominal pains and miscarriage (Akunwa et al., 2014). Heavy metals cause significant pollution especially for aqueous systems thus, in recent years, there has been increased research interest concerning the removal of heavy metal contaminants both in relation to improving existing methods and the development of new treatment approaches. Most of the studies in recent years for heavy metal removal is for the development of cheap and effective removal method. Especially in studies of the adsorption, to investigate the availability of waste products as adsorbent has become quite interesting. Coal fly ash is very interesting because it can be found as a waste material available in large quantities free of cost from by-product of thermal power plant used coal as fuel (Kuncoro and Fahmi, 2013). The use of fly ash in removing different contaminants from wastewater solves two problems: water quality and waste management (Cho et al., 2005; Moyo et al., 2012). The major constituents of fly ash are silica, alumina, ferrous oxide, calcium oxide and varying amounts of carbon. Fly ash is a strong alkali material, which exhibits pH of 10–13 when added to water, and its surface is negatively charged at high pH levels. Hence, it is expected that metal ions can be removed from aqueous solutions by precipitation or electrostatic adsorption (Cho et al., 2005). In the literature, there are reports of the effectiveness of fly ash waste in the removal of heavy metal ions (Alinnor, 2007, Bayat, 2002, Moyo et al., 2012, Musapatika et al., 2010, Vishwakarma et al., 1989), dyes (Kara et al., 2007, Sun et al., 2010, Visa et al., 2010, Wang et al., 2005) and organics (Kumar et al., 1987, Aksu and Yener, 2001) from aqueous solutions. Also, some studies reported in the literature with the pretreatment method such as using alkalis (Pengthamkeerati et al., 2010), acids salts (Gao et al., 2015) and mechanical activation (Kumar et al., 2005, Kumar et al., 2007, Kumar and Kumar, 2011, Patil and Anandhan, 2012) for improving the adsorption capacity of the fly ash.

Mechanical activation refers to wherein represent the effect of increasing the specific surface area in addition to increased dissolution. The latter appears to be the most advantageous of these pre-treatment processes since it offers the possibility of altering the reactivity of solids through physicochemical changes in bulk and surface without altering the overall chemistry of the material. Many studies showed that high energy milling creates an enlarged surface area that increases the reactivity of the fly ash (Kumar et al., 2005, Kumar and Kumar, 2011). But, an application on metal removal of these fly ashes has not yet available in literature.

The aim of this study is to measure and model the adsorption of Cu(II), Mn(II), Ni(II), Pb(II) and Zn(II) on raw and activated in planetary ball mill fly ashes. The activated fly ash was firstly used as an adsorbent for the removal of the heavy metals from aqueous solutions. In addition, the optimum removal conditions were identified for each metal ions by evaluating effective factors employing response surface method (RSM) as an experimental design method. The estimated model equations were graphically shown in the form of a 3-D surface plots and contour plots. Adsorption equilibrium system was defined by testing the applicability of Langmuir and Freundlich isotherms. Moreover, thermodynamic and kinetic parameters were calculated to determine the adsorption mechanism of each heavy metals onto both fly ash samples. This paper is a critical overview on chemical, structural, and morphological changes in fly ash properties with mechanical activation using high energy planetary ball mill.

Section snippets

Materials

In the present study, the raw fly ash samples obtained from Turkish (Kangal Power Plant) (Bingöl and Akçay, 2005) were used. Explaining the mechanism of adsorption processes utilizing fly ash samples have been aimed for the removal of heavy metals such as Cu(II), Mn(II), Ni(II), Pb(II) and Zn(II) from aqueous solutions. 1000 mg/L Cu(II), Mn(II), Ni(II), Pb(II) and Zn(II) stock solutions were prepared from salts, Cu(NO3)2·21/2H2O, Mn(NO3)2·4H2O, Ni(NO3)2·6H2O, Pb(NO3)2 and Zn(CH3COO)2·2H2O

Characterization of fly ash samples

Mineral composition of ash samples was determined by means of X-ray diffraction (XRD) analysis.

During mechanical activation, the crystalline material can become amorphous with grinding. The reduction in peak height is accompanied by a general broadening of the peaks, which means that the crystalline particle size decreases and the lattice strain increases (Xiaoru et al., 2008). Fig. 1 shows that mechanical activation causes visible changes in crystallinity degree of some phases (especially

Conclusion

Heavy metal pollution leads to serious environmental problems especially polluting water systems. In this study, the adsorption characteristics of fly ash as a waste product were investigated for the removal of some heavy metals such as Cu(II), Mn(II), Ni(II), Pb(II) and Zn(II) from aqueous solutions. Also, the effects of mechanical activation were examined on the adsorption capacity of fly ash.

The adsorption process applied to remove these ions from aqueous solutions was optimized successfully

References (42)

Cited by (63)

  • Removal of lead (Pb(II)) and zinc (Zn(II)) from aqueous solution using coal fly ash (CFA) as a dual-sites adsorbent

    2021, Chinese Journal of Chemical Engineering
    Citation Excerpt :

    In this sense, adsorption process can take place on both minerals and unburned carbon, thus enabling CFA to act as a dual-sites adsorbent [32,33]. Several research studies have reported the effectiveness of CFA in the removal of Pb(II) ion [27,34–36] and Zn(II) ion [34,36] from aqueous solutions. For improving the adsorption capacity of CFA, several modifications and activation processes have also been previously studied, such as modification using alkalis [35,37–39], and mechanical activation [34].

View all citing articles on Scopus
View full text