Effect of modification by five different acids on pumice stone as natural and low-cost adsorbent for removal of humic acid from aqueous solutions ‐ Application of response surface methodology
Graphical abstract
Introduction
Natural organic matters (NOM) in surface water resources are the most common amphiphilic compounds, most of which consists of humic substances [1]. Humic substances are soluble organic polyetherolite that are formed from the decomposition of plant and animal residues in aqueous environments [2,3]. Structurally, humic acid substances are a broad mixture of macromolecules with yellowish to black and generally heterogeneous appearance. They comprise carbon, oxygen, hydrogen and, occasionally, a low amount of nitrogen, phosphorus, and sulfur [4]. Their solubility in aqueous environments depends on the number of COOH and OH groups [5]. Humic supplies are separated into humin, humic acid (HA), and fluorocarbon acid based on their solubility under acidic or alkaline circumstances in aqueous solutions [6]. The proportion of HA as a critical component of humic substances can increase with the microbial degradation of biomolecules and may form a significant portion (40–90%) of the dissolved organic matter (DOM) in almost all water sources [7]. HA has aliphatic and aromatic features, and its molecular weight ranges between 500 and 250,000 [8].
The negative charge of the humic material can lead to an increased transportation of different metals and elements such as bromine that consequently results in an increased toxicity and intensification of toxic byproducts generation [9]. There is an upward concern about the adverse effects of HA on living organisms due to the persistent nature and high tendency to adsorbing various contaminations, comprising pesticides and heavy metals [10].
Humic substances should be removed from water treatment systems in different ways: (a) Producing disinfection by-products (DBPs) such as trihalomethanes by reacting with chlorine in water purification; (b) Increasing the transportation of hydrophobic organic pollutants or heavy metals by joining to them; (c) Causing bacteria to grow in water distribution systems being as food sources; and (d) Creating unpleasant odor and taste in drinking water [11]. In general, the concentration of humic substances in natural water was reported in the range of 1.0 to 10 mg/L [12]. Humic acid has been known as one of the primary precursors of disinfection by-products, especially trihalomethanes (THM) and halo acetic acid (HAA), which have adverse health effects comprising the potential for carcinogenicity and undesirable effects on renal, hepatic, neurological, and genital/reproductive tissues [13].
Commonly, conventional methods for removal of humic substances include coagulation [14], membrane filtration [15], ion exchange [16], advanced oxidation process (AOP) [17], and adsorption [18].
The coagulation process produces a large amount of sludge and high operating costs. Ionic exchange reestablishes high amounts of salt and hummus. The membrane processes produce liquid waste and the humic substances tends to block the membrane, which limits the use of membranes [19]. Adsorption is an acceptable method due to the simplicity, low operational cost and capability in eliminating acid folia in water. As a consequence, many adsorbents including activated carbon [5], resin [20], chitosan [21], iron oxides [22], and various forms of nanographene [2], graphene oxide [23], and other adsorbents have been employed to remove these substances from aquatic systems. In case that the use of the above adsorbents is essential to remove large-scale humic acid (e.g., in a municipal water plant), the provision, preparation, and recovery of most of these adsorbents are not nearly cost-effective. Thus, it is necessary to find adsorbents with high abundance, and low cost and, then, focus shall be made to improve the efficiency of this type of adsorbent via various modification methods.
One of the cheapest and most abundant adsorbents that have received considerable attention by environmental researchers to remove various pollutants from aqueous solutions is pumice stone. Pumice stone as a natural stone is a silica glass with a color that varies from bright to dark gray. It has a specific surface area between 28 and 54 m2/g, high porosity (85%), structurally with irregular cavities, dry weight of 500 to 800 kg per m2, Mohs hardness of 5–6, and 60 to 75 percentage of silica [24].
Pumice stone as a natural adsorbent was utilized for the removal of various pollutants including heavy metals [24], multiple colors [25,26], phenols and their derivatives [27], fluoride, nitrate [28], and ammonium [29]. Nevertheless, few studies have been carried out on the use of this adsorbent in the removal of humic acid [30]. Thus, further research is needed to characterize the use of this adsorbent in removing humic substances from aquatic environments.
The present study aimed to scrutinize the efficiency of pumice modification with five different acids (acetic acid, hydrochloric acid, phosphoric acid, humic acid, and nitric acid) in removing humic acid from aqueous solutions. Accordingly, a new method was used in this study for pumice adsorbent modification by using five different acids (acetic acid, chloride, phosphoric acid, humic acid, and nitric acid) to compare the impact of these acids on the adsorbent efficacy and also, surveying the effects of these acids on pumice structure for the humic acid removal. Additionally, response surface methodology (RSM) was employed to investigate the role of influential parameters (pH, adsorbent dose, contact time, and initial concentration of HA) on the process of adsorbing humic acid and to develop the optimal conditions for the operation.
Section snippets
Chemicals, equipment and adsorbent characteristics
Raw Pumice was obtained from Qorveh region in Kurdistan province, Iran. All chemicals of analytical grade including, NaOH (CAS 1310-73-2), HCl (CAS. 7647-01-0), H2SO4 (CAS 7664-93-9), HNO3 (CAS 7697-37-2), H3PO4 (CAS. 7664-38-2) and CH3COOH (CAS.64–19-7) were purchased from Merck (Darmstadt, Germany). The working solution of humic acid was prepared by proper dilution of its original stock using double distilled water (DDW) according to Part 2.3. (Preparation and purification of humic acid (HA)
X-ray fluorescence (XRF) analysis
The X-ray fluorescence analysis was employed to distinguish the structure of the adsorbent and changes in its formation due to the modification with different acids. Table 4 displays the XRF results of the raw and modified pumice. XRF can be utilized to identify interactions between components of pumice and different acids used to modify it. In this interaction, three possible states can be observed: a) some materials would be physically dissolved in acid solution, b) some of them may react
Conclusion
In this research, the adsorption process of humic acid by the modified pumice with five different acids (acetic acid, hydrochloric, phosphoric acid, sulfuric acid, and nitric acid) and the raw pumice was studied. The removal efficiencies for the absorbents were as H2SO4-MP > HNO3-MP > H3PO4-MP > HCOOH-MP > HCl-MP > Raw-P. Additionally, by increasing the adsorbent dose and contact time, the HA removal efficiency by both the raw and modified adsorbents was increased; while the removal efficiency
Acknowledgment
This research was part of an MSc degree thesis in the environmental health engineering and was financially supported by the grants of Tehran University of Medical Sciences (TUMS), Tehran, Iran (code: 97-03-27-39639), and (code: 97-02-27-38186).
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