Experimental

2.1. PREPARATION AND CHARACTERIZATION OF ACTIVATED CARBON FROM CO-MINGLED WASTES

The activated carbons were prepared from the mixture of co-mingled natural organic liquid and solid wastes. For the blending process, the components were combined in the proportion of 25% biomass, 25% low-grade coal, and 50% petroleum wastes (either aliphatic petroleum stock or aromatic tar stock).

All of the blend components were obtained from local sources in the Ukrainian Donbass region. Bituminous coal h8b was obtained from Lidievka mine, located in Donetsk (Eastern Ukraine).

The selected biomass was sunflower husks. Sunflower husks were graciously provided by the Oil Process Plan "Cargil" of Joint Ukrainian-USA Oil Corporation in Donetsk.

Petroleum feedstock components were courtesy of Coke Process Plan "Avdeevka", located in Donetsk, and of Odessa Harbor's Unloading Petroleum Station. The aliphatic/aromatic nature of the petroleum components was evaluated by 1H-NMR aromaticy monitoring.

The feedstock from Coke Process Plan "Avdeevka" had 40.2% of aromatic fraction; in the present study it is referred to as "aromatic tar stock". The petroleum feedstock from Odessa Port's Unloading Petroleum Station is referred to as "aliphatic petroleum stock", due to the high content (up to 89.2%) of branched hydrocarbons.

In order to gain some insight into the reactions which may occur between the components of the co-mingled waste during the activation process, the pyrolytic behaviour of the single components and of their blends was investigated by thermogravimetric analysis (TG/DTG) using a Perkin-Elmer TGA 7 thermal analyser. The heating rates varied between 5°C/min and 20°C/min, and the samples were heated up to 1,273 K under inert atmosphere. The thermogravimetric parameters, such as the temperature of initial weight loss (?mit), the temperature of final weight loss (7f-mal), the temperature of maximum rate of weight loss (Tmax), the rate of weight loss (mg/min) and the carbon yield at 1,223 K (%), were determined from the TG and DTG data.

For the activated carbon synthesis, either direct high-temperature activation with steam at 1,123 K or activation via a step of low-temperature carbonization at 623 K under inert atmosphere, were adopted. The soaking time of both processes was varied between 1 and 3 h. In order to avoid coke formation during the activation process, 5% (from weight of the blends) of additives of binary eutectic of Na/K carbonates was used.

The structural parameters of the produced carbons were evaluated by nitrogen adsorption at 77 K and temperature-programmed desorption. The experiments were carried out using Micromeritics ASAP 2010 and Micro-meritics TPD/TPR 2900 equipments. The degree of heterogeneity of the samples was visually assessed by Scanning Electron Microscopy (SEM); the topography of the surface was also studied by high-resolution Auger Depth Profiling with PHI SAM-660 equipment.

2.2. ANALYSIS OF THE HEAVY METALS ADSORPTION FROM TECHNOLOGICAL SOLUTIONS

Real multi-component aqueous solutions, proceeding from "Pridneprovskij Plan of Color Metals" State Enterprise (Power Ministry of Ukraine), were used as a source for copper (II), cobalt (III), nickel (II), iron (II), and manganese (II) chlorides.

The adsorption process was analyzed in batch mode using as adsorbent the parent activated carbon from the co-mingled wastes and also its oxidized form, which was obtained by treating the parent carbon with 1M HNO3 (reflux) during 6 h at 90°C.

Sorption-kinetics experiments were carried out to evaluate the specific rate constants of the simultaneous adsorption of the 3-d transition metals on the activated carbons with different surface oxygen functionalities.

Kinetics studies were carried out at fixed initial pH of 3.6 of the wastewater source, without adding any buffer to control the pH to avoid an additional electrolyte in the system.

The carbon loading was fixed at 5 g/L and the metal concentrations in the real solution were 11.4 mg/L of copper (II), 9.4 mg/L of cobalt (III), 9.8 mg/L of nickel (II), 11.07 mg/l of iron (II) and 9.5 mg/L of manganese (II). The process was carried out at constant temperature (297 ± 1 K).

To enhance the adsorption rate, the carbon/metal solutions were stirred on a shaker at 160-180 rpm. The adsorption time was varied between 1 and 72 h.

At the end of the experiments, the adsorbent was removed by filtration through membrane filters with a pore size of 0.45 ^m. The metal equilibrium concentration in the aqueous solution was measured by atomic adsorption spectroscopy with AA-6300 Flame Emission equipment, according to standard procedures [22].

The equilibrium amount of adsorbed metal on the activated carbon was quantified by mass balance. The following parameters were used: adsorption capacity expressed as amount of metal adsorbed per unit mass of adsorbent, i.e. Me uptake (mg/g); and sorption efficiency of the system, determined as the percentage of metal ions removed relative to their initial amount, i.e. Merem (%). The specific rate constants for the heavy metal adsorption were found from the Lagergren equation [23]:

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