Замърсяването с тежки метали е един от най-сериозните проблеми засягащи околната среда. То се отдава главно на минната индустрия и поради тази причина е основен проблем в развитите страни

Heavy metal pollution is one of the most pressing problems presenting a challenge in engineering a cost effective as well as efficient process for their removal from contaminated waters. Pollution with heavy metals is a problem emerging in industrially developed countries with well pronounced mining industry. Heavy metals are usually a waste accompanying the process of metal extraction, mineral processing, metal plating. Although there isn’t a uniform (unanimous) definition of heavy metal it is usually associated with elements with metallic properties and high atomic weight. Heavy metal ions are well known for their toxicity, both acute and chronic as well as their carcinogenicity. Their release in environment can pose a serious health hazard due to their tendency for bioaccumulation and biomagnification (Paulino et al., 2006; Borba et al., 2006; Oyaro et al., 2007).

A lot of methods for heavy metal ions removal were developed including chemical precipitation (Huisman et al., 2006), ion-exchange (Petruzzelli et al., 1995), membrane filtration (Shaalan et al., 2001; Landaburu-Aguirre et al., 2009), reverse osmosis (Mohsen-Nia et al., 2007; Muthukrishnan et al., 2008), dialysis (Mohammadi et al., 2005), adsorption (; Mohan et al., 2006; Liang et al., 2010; Liu et al., 2011), electrochemical methods (Roundhill et al., 2002) and etc. Recently a promising new method for a water treatment has been developed – photocatalysis (Lazar et al., 2012; Nakata, K. et al., 2012). Although there are a large number of research papers in the past ten years regarding the advantages and the prospects of this method it is still not employed as an industrially applied water treatment process.

Each of the above listed methods has its advantages and disadvantages regarding their cost, efficiency and selectivity towards heavy metal removal from wastewaters. Adsorption as a process for water purification is probably one the most widely applied worldwide due to its cost efficiency, versatility and high removal efficiency. Different types of adsorbents have been applied for water purification – both synthetic and naturally occurring. Amongst the ones that have attracted the attention of researchers due to their excellent qualities are zeolites (El-Kamash et al., 2005; Kocaoba et al., 2007; Jamil et al., 2010; ), activated carbon (Kang et al., 2008), clays (kaolinite and montmorillonite: Bhattacharyya et al., 2008) as well as polymer-based materials (Liu et al., 2010; Kiani et al., 2011; Ma et al., 2012). Activated carbon is meso- and microporous material obtained mainly from wood and coal. It can be activated by chemical or physical methods and depending on that the properties of activated carbon can vary significantly. The origin and the type of activation determine the specific area, pore size and volume, as well surface functionality. It is one of the most widely used sorbent due to his high sorption capacity combined with its versatility and cost efficiency.

Norit CA1 is chemically activated carbon (with phosphoric acid) with highly developed surface. Depending on the method of activation there can be a different type of functional groups on the activated carbon surface. This is of significant importance when one uses the activated carbon (AC) for heavy metal ion removal. In that case the predominant is electrostatic interaction between the functional groups on surface of AC and the metal ion. Norit CA1 is referred as so called L carbons (low temperature activation), which is associated with the formation of acidic groups on their surface and negative zeta potential as well as their ability to adsorb metal ions (Corapcioglu et al., 1987). This interaction is dependent on the pH of the media, ionic strength and temperature.

Here is presented the pH and ionic strength dependence of the adsorption of Cd(II) and Zn(II) ions onto Norit CA1. Four different pH media with two different ionic strength were tested. The results from obtained from adsorption of the metal ions were processed using the linear form of Freundlich isotherm, where q is the mass concentration of the adsorbed metal ion per mass of sorbent (q) and its concentration in the solution at equilibrium (C_M).
(1)

Usually parameter K (intercept) is associated with the sorption capacity of the material while 1/n (slope) is connected to the sorption rate.

As it is shown on Figure 2 the adsorption capacity of Cd(II) ions onto Norit CA1 is practically not affected by the pH media (with slight exception of pH 7.8-mineral water). The parameters K and 1/n are the same for the pH 2.7; 4.8 and 6.5, meaning that Cd(II) are adsorbed with the same rate and amount onto Norit CA1 at that conditions.

Here on Figure 3 is shown the corresponding dependence of the adsorption of Zn(II) ions onto Norit CA1 at different pH. One can see from it that there is no apparent effect of pH onto adsorption properties of Zn(II) onto the AC. They are practically indiscernible.

By comparing the results obtained from the linear plots of Freundlich isotherms for adsorption of Cd(II) and Zn(II) ions (Figure 2 and Figure 3) onto Norit CA1 one can summarize that there is not any well pronounced dependence of the adsorption from pH media, both for Cd and Zn ions. This can be attributed to the chemical modification of activated carbon with phosphoric acid. That allowed the formation of a surfaced with relatively strong acidic groups that doesn’t affect from pH of the media in a tangible way. That makes Norit CA1 a very attractive choice when one needs to remove heavy metal ions, because it gives the opportunity to adsorb the same amount of metal ions regardless of pH of the media.

There is no significant difference in the adsorption capacity and rate regarding Zn(II) and Cd(II) ions. This means that Norit CA1 is not selective adsorbent, at least not in respect these two metal ions. But this fact also makes it a versatile adsorbent for different types of substances.

The fact that adsorption in this case is mainly due to the electrostatic interaction between metal ions and AC surface makes it preferable to establish the effect and the ionic strength dependence since it is expected to be one of the parameters which will influence the adsorption capacity.

On Figure 4 data are presented for adsorption of Cd(II) and Zn(II) ions onto Norit CA1 ot pH=4.8 and ionic strength – 0.1M and 0.01 M. Here can see the way that adsorption is affected by ionic strength of the media. In the case of Cd(II) ions when we increase the ionic strength from 0.01M to 0.1M the adsorption capacity and rate does not change significant which is also the case of Zn (II) ions adsorption. Overall we can observe from Figure 4 that ases as expected while the adsorption rate (slope) does not change significantly. In the case of we observe that Zn(II) ions have higher adsorption capacity onto Norit CA1 than Cd(II) at the same other conditions.

Activated carbon is one the most widely applied sorbent for water and wastewater treatment. That is due to its high sorption capacity, chemical resistance and versatility. It can be successfully applied for removal of both organic and inorganic substances (pollutants).

The experiments conducted in this research revealed that activated carbon Norit CA1 is very suitable for heavy metal ions removal from waters and wastewaters. The results has shown that there is not any significant decrease of adsorption capacity at different pH media. That makes Norit CA1 useful for wastewater treatment since the low pH will affect much the adsorption of heavy metal ions onto surface of the activated carbon. The other fact that recommends it is versatility – it was shown that adsorption capacity of Zn(II) ions was as high as the Cd(II) ions. This leads us to believe that it can remove a large number of different metal ions from polluted waters with the same efficiency.

Overall activated carbon (in this particular case Norit CA1) has shown excellent adsorption properties to the tested heavy metal ions, one of his major advantages being the non-pronounced pH dependence as well as its versatility.

The authors thank to project MTF-120/ 07.07.2013 of UMG “St. Ivan Rilski” for the financial support and to IPC-BAS for SEM images of Norit CA1.

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