1. Introduction
The abundance of arsenic (As) in the Earth’s crust is around 1.5–3 mg/kg, making it the 20th most abundant element and a component of more than 245 minerals [
1]. When the groundwater’s pH and bicarbonate anion (HCO
3−) concentration are high, As is easily dissolved and passes to the groundwater cycle [
2]. As toxicity affects all body systems, causing both acute and chronic poisoning [
3]. Acute exposure is rare and happens mostly with exposure to arsenite (As
III) than arsenate (As
V) [
4]. Long-term exposure leads to a variety of illnesses known as arsenicosis [
5], which includes skin, bladder, kidney, and lung cancer, along with black foot disease [
6]. As
III is more toxic than As
V [
7] and also harder to remove from water [
8].
With nearly 1 billion people exposed to arsenic by food, and more than 200 million people exposed to it via drinking water [
9,
10,
11], As is a serious threat to the physical, social, and economic well-being of people, affecting especially the population of develo** countries [
12]. Countries affected by high arsenic concentrations in groundwater include Argentina, Australia, Bangladesh, China, Chile, Mexico, India, New Zealand, Nepal, USA, Vietnam, and Taiwan [
13].
In addition, the typical and harmful pollutants in develo** countries are of biological origin, as diseases present in almost 50% of their populations are associated with water, both for supply and sanitation [
14]. Worldwide, according to estimations, safe drinking water is unavailable for 1.1 billion people; water scarcity is suffered by 2.7 billion people, and 5 million people die each year due to waterborne infections [
15,
16]. These infections can be caused by viruses, bacteria, or protozoa [
17].
The World Health Organization (WHO) recommends that the As amount in water should not surpass 10 µg/L [
18]; as for microbiological water quality, the WHO recommends the use of the following microbial water quality indicators: total coliforms, thermotolerant coliforms,
Escherichia coli, intestinal enterococci, enteric virus, and coliphage virus (none of which should be detected in drinking water) [
19].
The growing population and climate change are two of the main factors that are increasing the demand for drinking water; it is then a priority to research drinking water treatments to improve processes in terms of reliability, efficiency, safeness, and ease of implementation [
20]. Additionally, economic feasibility, technical viability, and environmental safeness must be complied with for a technology to be considered sustainable [
21].
Recently, the scientific community has shown extensive interest in advanced oxidation processes (AOPs), considering them as the most promising technologies for the potabilization of water and the treatment of wastewater on account of the nonselectivity of reactive oxidizing species (ROS), enabling AOPs to remove pollutants, including microbes and organic and inorganic contaminants [
22].
Heterogeneous photocatalysis (HP) with semiconductors (or photocatalysts) is an AOP developed in 1972 with several advantages, including its ability to use solar light and that fact that is environmentally friendly and has a relatively low cost [
23,
24,
25]. When the photocatalyst is irradiated with light whose energy is higher than the photocatalyst bandgap energy level, electrons in the valence band (VB) migrate to the conduction band (CB), generating a positive hole (h
+) and an extra electron (e
–) in the VB and CB, respectively [
26]. Oxygen adsorbed on the photocatalyst surface can react with e
– to form superoxide radicals (O
2●−), while water can react with h
+ to generate hydroxyl radicals (HO
●) [
27].
HP with titanium dioxide (TiO
2) has been investigated for As oxidation, which is a good approach as As
III is found as a neutral charge oxyanion in a wide pH range, and, in contrast to other oxyanions, adsorption onto metal oxides or clays is inefficient, and precipitation at near neutral pH barely occurs [
28]. Although As
V is a triprotic acid and can be found in several forms depending of the medium pH, its removal from water is easier with processes such as chemical precipitation [
29] and adsorption [
30].
For the last two decades, HP has also been widely investigated for water disinfection, showing potential for treatment through oxidative stress caused by ROS, Gram positive and negative bacteria, DNA and RNA viruses, and even algae [
31]. ROS can attack cell membrane components, altering cell integrity, which results in a cytoplasm leakage [
32]; they can inhibit required cell activities such as protein synthesis [
33]; they can also break organic covalent bonds present in biomolecules [
34]. Many factors affect the efficiency of disinfection via HP, including the chemical nature and concentration of the microorganisms, time of treatment, light intensity, water matrix, deficiency of atomic ligands, surface energy level, photocatalyst properties, and solution pH [
22].
One of the main drawbacks limiting commercial and industrial HP application is the lack of reactor designs efficient enough to handle large volumes of water [
35]. Many types of reactors have been studied and developed, but standard procedures for scale up are still lacking; HP technology readiness level (TRL), which ranges from TRL = 1 (proof-of-concept stage) to TRL = 9 (full operational scale stage), is between TRL = 4 (lab scale validation) and TRL = 5 (ongoing pilot scale applications) [
36]. Other relevant issues concerning reactor design, such as reducing photon and mass transfer limitations [
37] or a thorough understanding of heat, mass, and light transfer in the system [
38], are a current research interest as are operation conditions, such as analyzing reactor performance with real water matrixes instead of synthetic water matrixes [
39].
In this work, heterogeneous photocatalytic arsenic oxidation and water disinfection were explored in two types of solar reactors, a compound parabolic collector reactor (CPC) and a flat plate reactor (FPR); a real groundwater matrix was used, and coliform disinfection was also analyzed. As removal via chemical precipitation with ferric chloride (FeCl3) was explored, following oxidation. Estimations of the collector area per order (ACO) (m2/m3-order) were performed for the evaluation of the area or energy requirements by every reactor. The results were also analyzed from a reaction kinetic and statistical standpoint.
4. Conclusions
The treatments CPC–HP + H2O2 and FPR–HP + H2O2 yielded the best oxidation for AsIII, with rates around 90%. These treatments also exhibited the highest oxidation reaction rate constants, with 6.8 × 10−3 min−1 and 6.7 × 10−3 min−1, respectively.
As removal rates achieved via chemical precipitation for the aforementioned treatments were 98.7% and 98.6%, reaching the As concentration level recommended by the WHO, which is below 10 µg/L.
Additionally, no coliforms were detected in the irradiated treatments, which adds up to the advantages of HP as a potential and promising technology for water potabilization and wastewater treatment.
The determination of ACO showed that CPC was on average 30% more efficient than the FPR, requiring less photocatalyst-covered area.
The effects of AOP, H2O2 addition, and light irradiation were statistically significant for the AsIII oxidation reaction rate, while the type of reactor utilized, spiking with MWTE, or fluence were not (p < 0.05), as found out with an ANOVA.