Raw water is the main resource for industrial water usage for washing, fabricating, processing, diluting, boiling and cooling products. Raw water includes rainwater, groundwater, and water from rivers and lakes. This water is not directly used due to the presence of contaminants, and pre-treatment of raw water is essential [1]. Aluminium sulphate (Alum) and other coagulants, such as polymers, are generally used to coagulate the fine suspended solids in raw water by forming a mass large enough to settle and enable removal [2]. This water treatment residue (WTR) consists of silt, sand, clay, organic substances, and undesirable components with a very high water content of up to 80% [3]. In Thailand, 137,900 tonnes of WTR is generated annually and is conventionally disposed of in landfill [4]. Attempts have been made to use WTR as a replacement for fire brick clay since its properties are similar to clay. The addition of 10% WTR with a firing temperature above 1000°C has been shown to obtain good properties of water absorption and mechanical resistance [5] However, using a large amount of WTR as a clay replacement in ceramic production is problematic due to its seasonally dependent availability [4, 5].
Groundwater is a major resource for tap and drinking water. However, it contains iron and manganese contaminants due to leaching from the laterite soil layer after heavy rainfall with alternating wet and dry periods [6]. In particular, the presence of iron can contaminate water, resulting in a reddish-brown colour and undesirable taste and odour. The dissolved, colourless form of iron, Fe(II), is converted into an insoluble form, Fe(III), in contact with air. In groundwater, iron exists in Fe(II) form due to the lack of oxygen. When exposed to air in a pressure tank or the atmosphere, a cloudy and reddish-brown substance is formed [5, 6]. In addition, dissolved manganese may form black particulates in water. These substances can deposit in the water pipe and subsequently flow through the tap, leading to reddish-brown stains on tableware and laundry that are difficult to remove. Iron and manganese removal systems are therefore required to overcome this problem. Many advanced materials are available to remove iron from water; for example, nano bentonite [7], microporous chitosan blend membrane [8], and carbon nanotubes [9]; however, some technologies are expensive and not cost-effective for water treatment systems.
Several studies report the use of waste ash in a geopolymer, reflecting its suitability and availability [10, 11]. Alkaline solutions and heat curing are used to activate the reaction between the active alumina (Al2O3) and silica (SiO2) in the ash, leading to the formation of Si-O-Al bonds in an alumino-silicate product. Geopolymeric materials are the alternative choice for recycling industrial waste [12] and are often used as materials for roadway construction and pavements due to their high disposal rate [13. 14, 15]. Apart from waste ash, natural clay and calcined clay have also been widely used as source materials for geopolymer synthesis [16, 17, 18]. Clay is rich in Al2O3 and SiO2, and is classified as an alumino-silicate material and is normally calcined to improve its reactivity. Generally, geopolymer materials are used as construction materials; however, since WTR has a high content of Al2O3 and SiO2 and contains clay, porous geopolymers have recently been proposed as a CO2 absorbent [19], acidic-wastewater treatment [20], and heavy metal remover [21, 22]. Therefore, this research proposes the use of WTR as the source material for geopolymer production. Given the free alkali content of the geopolymer framework, the OH-releasing behaviour of a WTR-based geopolymer was investigated based on the precipitation of Fe(II) ions. The characteristics of Fe(II) removal were studied through heating a WTR-based geopolymer. This alternative use of WTR in forming a geopolymer, and its use for iron removal from groundwater, is simple and cost-effective.