## 1 Introduction  
In landfill leachate from old landfill sites dominates chemical compounds of rather high molecular masses (e.g., humic substances (HSs)). Such leachates have lower TOC, COD, BOD\({}_{5}\) values and BOD\({}_{5}\)/COD ratio in comparison to those from young landfill sites. What is more, organics in mature leachates are rather refractory to biological treatment and could possibly be hazardous. In most cases, only pre-treatment methods, such as coagulation-flocculation, adsorption and biological treatment, are applied to landfill leachates (LL). Those methods are not sufficient in the case of mature LL, and more powerful methods should be recommended like membrane methods or advanced oxidation methods. The high efficiency of AOPs is related to the activity of hydroxyl radicals with a high redox potential (2.8 V). In recent years, the internal microelectrolysis (IME) process, which is derived from the heterogeneous ZVI/H\({}_{2}\)O\({}_{2}\) process [1], has attracted much interest. The idea of the process is based on the formation of galvanic microcells as a result of the difference in potential between the microelectrode materials, i.e., cathode (activated carbon (AC)) and anode (iron chips), in an acidic electrolyte solution [1; 2; 3; 4; 5]. The electrode reactions on the microcells are presented by the following reaction equations:  
\[\text{Anode:}\quad\text{Fe}^{0}\rightarrow\text{Fe}^{2+}+2\text{e},\qquad\qquad \text{E(Fe}^{0}/\text{Fe}^{2+})=0.44\text{ V} \tag{1}\]  
\[\text{Fe}^{2+}\rightarrow\text{Fe}^{3+}+\text{e}\qquad\text{E(Fe}^{2+}/\text{ Fe}^{3+})=-0.77\text{ V} \tag{2}\]  
\[\text{Cathode:}\ 2\text{H}^{+}+2\text{e}\rightarrow\text{H}_{2}\text{ (g)}\qquad\text{E(}2\text{H}^{+}/\text{H}_{2})=0.00\text{ V} \tag{3}\]  
In an acidic environment and anaerobic conditions, hydrogen is created at the cathode; however, it is contrary in aerobic conditions--the oxygen atom becomes an electron acceptor instead of the hydrogen atom, and the hydrogen peroxide is formed [1; 2; 3; 4; 5; 6]  
\[\text{O}_{2}\text{(g)}+4\text{H}^{+}+4\text{e}\to 2\text{O}^{ \bullet}+4\text{[H]}\to 2\text{H}_{2}\text{O}\quad\text{E (O}_{2}/\text{H}_{2}\text{O})=1.23\text{ V} \tag{4}\]  
\[\text{O}_{2}\text{(g)}+2\text{e}+2\text{H}+\rightarrow\text{H}_{2}\text{O}_{2 }\qquad\quad\text{E(O}_{2}/\text{H}_{2}\text{O}_{2})=0.69\text{ V} \tag{5}\]  
In neutral or alkaline conditions, an unfavorable reaction occurs at the cathode (6) [4]:  
\[\text{O}_{2}+2\text{H}_{2}\text{O}+4\text{e}\to 4\text{OH}^{-}\qquad \quad\text{E(O}_{2}/\text{OH}^{-})=0.40\text{ V} \tag{6}\]  
The hydrogen peroxide formed in reaction 5 immediately reacts with the iron cations Fe\({}^{2+}\) released in the oxidation process of the iron microelectrode and initiates the Fenton reaction (reaction 7) [2]. \[\text{Fe}^{2+}\text{ (aq)}+\text{H}_{2}\text{O}_{2}\text{(aq)}\rightarrow\text{ Fe}^{3+}\text{ (aq)}+\text{OH}^{-}\text{ (aq)}+\text{OH}^{\bullet} \tag{7}\]  
In the IME method, the anode material iron or cast-iron chips are used, while as the cathode, the AC in granulated (GAC) or powder form is used. The efficiency of the IME process depends on many factors, such as pH or the number of cathode and anode centers (microcells), i.e., the dose of microelectrodes and their mass ratio (Fe/GAC). Important parameters are the surface and granulation of the microelectrodes--fine-grained iron materials are more susceptible to dissolution. The use of carbon and iron material with similar granulation facilitates both the formation of microcells and the accessibility of impurities to the microelectrode surface. Although the IME process was used to treat wastewater from various industries, there [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27] are only few papers devoted to the issue of LL treatment with the IME process (Table 1) [1; 2; 3; 4; 5; 28; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34].