Fly ash superficially modified with humic substances from the Megalopolis lignitic power plant was prepared and evaluated for agricultural uses. UV-vis spectrophotometry and IR spectroscopy revealed that fly ash shows high sorption efficiency towards humic substances. Adsorption proceeds stepwise via strong Coulombic and hydrophophic forces of attraction between guest and host materials. Langmuir, Freundlich, BET, Harkins-Jura, and Dubinin-Radushkevich isotherm models were employed to evaluate the ongoing adsorption and shed light to the physicochemical properties of the sorbent-adsorbate system. Humic substances desorption and microbial cultivation experiments were also carried out to examine the regeneration of the humates under washing and explore the possibility of this material acclimatizing in real soil conditions, both useful for biofunctional agricultural applications. 1. Introduction Fly ash is an amorphous mixture of ferroaluminosilicate minerals generated from the combustion of ground or powdered coal at 400– C and belongs to the coal combustion by-products in power plants produced from bituminous, subbituminus, and lignite combustion. Fly ash is the mineral residue consisting of small particles that are carried up and out of the boiler in the flow of exhaust gases and are collected from the stack gases using electrostatic precipitators, flue gas desulphurization systems, and bag houses [1]. Approximately 70% of the by-product is fly ash collected in electrostatic precipitators, which is the most difficult to handle [2]. This fact pinpoints the necessity for environment-friendly uses of fly ash. Fly ash is mostly used as a substitute for Portland cement in manufacturing roofing tiles and as structural fill, sheetrock, agricultural fertilizer, and soil amendment [3, 4]. Chemically, 90%–99% of fly ash is comprised of Si, Al, Fe, Ca, Mg, Na, and K with Si and Al forming the major matrix. The mineralogical, physical, and chemical properties of fly ash depend on the nature of parent coal [5, 6]. All these applications are based on the presence of basic mineral elements resembling earth’s crust, which makes them excellent substituent for natural materials. The Greek peaty lignite of the Megalopolis Basin, formed during the Quaternary period and comprising significant quantities of humic substances and inorganic content [7, 8], may be an effective raw material for obtaining both humic substances and fly ash. During the last fifty years, Megalopolis lignite has been almost solely utilized for power generation producing solid wastes such as fly ash,
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