Arsenic-contaminated water is a severe problem in many regions including Bangladesh, West Bengal in India, Pakistan, Vietnam, China, Spain, Mexico, Chile, Argentina, and the United States [1
]. Arsenic in groundwater often originates from natural deposits in rocks, sediments, soils, and geothermal resources [2
]. Arsenic in water may also be related to arsenic use in wood preservatives, pesticides, herbicides, alloys, pigments, and pharmaceuticals [3
]. Arsenic exists in both inorganic and organic forms, while organoarsenic compounds are often absent or in very low concentrations in natural waters [4
]. The most common oxidation states of inorganic arsenic are −3, 0, +3, and +5, which form various species. Arsenite (+3) and arsenate (+5) are the primary oxidation states in aqueous environments, with arsenate predominant in surface waters and arsenite prevalent under anaerobic conditions, e.g., groundwater [5
]. Long-term exposure to low concentration of arsenic in drinking water could result in severe health effects and even cause cancer [6
]. The United States Environmental Protection Agency (USEPA) has set an enforceable maximum contaminant level (MCL) for arsenic in drinking water at 10 μg/L (10 parts per billion, ppb).
Arsenic cannot be easily degraded in solutions, but can be separated from water or transformed into insoluble forms by a variety of physicochemical processes, such as coagulation [7
], membrane separation [9
], ion exchange [11
], liquid–liquid extraction [12
], and sorption [13
]. Selection of arsenic treatment methods is primarily determined by cost, operational complexity of the technology, and disposal of residuals containing arsenic [7
]. The most common method for arsenic removal is coagulation and flocculation. Iron and aluminum salts are widely used as coagulants in conventional water treatment plants [14
]. High removal efficiency was reported and up to 97% of arsenic removal can be achieved when using iron salts as coagulant [15
Selective removal of arsenic from aqueous solutions with high salinity is required when membrane desalination processes, such as reverse osmosis [16
], nanofiltration [17
], and electrodialysis [10
], are used to treat arsenic-contaminated water. Arsenic concentration in desalination concentrate increases with increasing water recovery (ratio of product to feed water). Disposal of the desalination concentrate with elevated concentrations of contaminants is an increasing challenge for the implementation of desalination techniques, especially for inland facilities that have limited disposal options [18
]. Arsenic removal from desalination concentrate by the most common coagulation method using iron salts requires higher chemical demand than that in lower salinity water. In RO concentrate, Xu et al. found a Fe:As molar ratio of greater than 410 was required in order to achieve 80% arsenic removal using ferric chloride due to the high salinity and ion competition [22
Sorption is a promising method that reduces chemical demand and the amount of sludge for disposal [23
], and is attractive for small-scale treatment systems due to ease of operation [24
]. A variety of sorbents have been utilized for arsenic removal, including ferrihydrite [25
], hydrous zirconium oxides [26
], hematite [27
], and goethite [28
]. For sorption processes, alternative sorbents that are cost-effective and environmentally friendly are needed. A low-cost and effective substitute for arsenic sorbents could be drinking water treatment solids (DWTS), the residuals produced from coagulation and flocculation processes during water treatment, which often utilize aluminum- or ferric-based coagulants [29
More than two million tons of DWTS are produced every day in the United States [30
], which are mainly disposed through landfills and can be used as a viable substitute to commercial sorbent for arsenic removal. Toxicity characteristic leaching procedure (TCLP) testing revealed a low risk of contaminants leaching from DWTS, indicating that most DWTS could be a safe sorbent [31
]. Laboratory studies demonstrated that contaminants such as phosphorus [33
], hydrogen sulfide [34
], metals [35
], fluoride [36
], and arsenic [22
] had a strong sorption affinity for DWTS. Equilibrium studies showed that both iron- and aluminum-based DWTS had a high capacity for arsenic, reaching a sorption capacity of about 15 grams arsenic per kilogram DWTS [37
Although previous studies have demonstrated the feasibility of arsenic removal using DWTS, there is a substantial knowledge gap with respect to the removal efficiency of arsenic from desalination concentrate with high salinity. Therefore, the focus of this study was on investigating the DWTS sorption of arsenic from RO concentrate under different operating conditions, aiming to understand the kinetics and interactions between arsenic and DWTS. Batch experiments were conducted to elucidate the impact of contact time, pH, sorbent dosage, initial arsenic concentration, salinity, and natural organic matter on arsenic sorption from RO concentrate.
2. Materials and Methods
2.1. RO Concentrate and Analysis
The RO concentrate was collected from the Kay Bailey Hutchison Brackish Groundwater Desalination Plant in El Paso, Texas. The total dissolved solids (TDS) concentration of the RO concentrate was 10 ± 2.3 g/L. The major ions in the RO concentrate included sodium (2660 ± 368 mg/L), calcium (673 ± 113 mg/L), potassium (69 ± 5.4 mg/L), magnesium (168 ± 22 mg/L), chloride (4993 ± 656 mg/L), and sulfate (1272 ± 226 mg/L), while minor ions included manganese (332 ± 8.6 µg/L), and arsenic (63 ± 11 µg/L). The pH of the RO concentrate was 7.8 ± 0.4, the alkalinity was 388 ± 6 mg/L as CaCO3, and the dissolved organic carbon (DOC) concentration was 4.1 ± 2.6 mg/L. The brackish groundwater was chlorinated before RO process for biofouling control; therefore, the arsenic in the RO concentrate was oxidized to arsenate.
Electrical conductivity and pH of the water samples were measured using a conductivity and pH meter (Model 431-61, Cole-Parmer, Vernon Hills, IL, USA). DOC was quantified using a carbon analyzer (Shimadzu TOC-L, Kyoto, Japan). Common anions were measured using an ion chromatograph (IC, ICS-2100, Dionex, Sunnyvale, CA, USA), and the concentrations of trace metals and metalloids were quantified using inductively coupled plasma mass spectrometry (ICP-MS, Elan DRC-e, PerkinElmer, Waltham, MA, USA). Alkalinity was measured using a digital titrator (Hach, Loveland, CO, USA) and 1.6 N sulfuric acid standard solutions to a pH 4.3 endpoint. The TDS concentration was measured following the evaporation method at 180 °C after filtering the RO concentrate sample using a 0.45 μm cellulose acetate membrane filter (Toyo Roshi Kaisha, Ltd., Tokyo, Japan). The fluorescence excitation-emission matrices (F-EEM) of filtered water samples were analyzed by a spectrofluorometer (Aqualog-UV-800C, HORIBA Jobin Yvon, Edison, NJ, USA).
2.2. DWTS and Characterization
The filter backwash DWTS used in the experiments were collected from the sand drying beds in a groundwater treatment plant in El Paso, Texas. The ground samples were then sieved into different particle sizes: <0.2, 0.2–0.4, 0.4–0.8, 0.8–2, and 2–5 mm. After microwave acid digestion of the solids, the elemental composition of the solids was analyzed by IC and ICP-MS [21
]. The organic content was analyzed by calcining the DWTS at 550 °C for 8 h in a muffle furnace (Furnace Vulcan 3-550, Dentsply International Inc., York, PA, USA). The moisture content was measured by the standard thermal evaporation method in an oven (OF-01E, Jeio Tech, Daejeon, Korea) at 105 °C for 24 h. The specific surface area of the solids was quantitated by an extended pressure adsorption analyzer (ASAP 2050, Micromeritics Instrument Co., Norcross, GA, USA). The salt titration method was adopted to determine the pH at the point of zero charge (pHPZC
) to investigate the impact of DWTS surface charge on arsenic sorption [38
of the DWTS was 6.9, indicating the transition point of net surface charge from positive to negative. The water and organic contents of the DWTS were approximately 7% and 10% of dry solid weight, respectively. The major metals in the DWTS included Al, Ca, Mg, K, Fe, and Mn. The detected trace inorganic constituents included Cu, Cr, Pb, As, Se, Ni, Zn, and Cd [21
]. It should be noted that iron was not the dominant element in the DWTS, although the treatment plant uses ferric chloride as the coagulant. The higher amount of aluminum in the DWTS was attributed to aluminosilicate minerals (in silt, sand, and clay) retained during coagulation/flocculation, sedimentation, and filtration processes.
No significant differences were observed for the DWTS with different particle sizes in terms of the elemental composition, specific surface area, moisture content, and organic content. Therefore, DWTS with 0.2–0.4 mm size was used in the experiments.
Batch leaching tests were conducted to quantify leaching of constituents from the DWTS using deionized water and RO concentrate [21
]. Trace amounts of As, Cr, Cu, Fe, Ni, and Se were detected in both deionized water and RO concentrate leaching solutions. Because of the high ionic strength in RO concentrate, more heavy metal cations desorbed or dissolved from the DWTS in RO concentrate than in deionized water. The high ionic strength in the RO concentrate decreased ion activity, which resulted in increased ion concentrations in the solution for a given activity in equilibrium with the DWTS, thereby facilitating the dissolution of the solids, and therefore higher heavy metal cation concentrations released from the DWTS in the RO concentrate. On the contrary, lower concentrations of oxyanions were released in RO concentrate, showing even increased net sorption of As and Se from RO concentrate, indicating the strong affinity of certain oxyanions for the DWTS. F-EEM spectra revealed the leaching of humics from the DWTS in deionized water, but the organic leaching in RO concentrate was one to two orders of magnitude less than in deionized water.
2.3. Sorption Experiments
Batch equilibrium experiments were conducted to investigate the arsenic sorption process at room temperature (23 ± 0.5 °C). The average arsenic concentration in the RO concentrate was 63 µg/L, six times higher than the arsenic MCL in the USEPA Primary Drinking Water Standards. To investigate the impact of initial arsenic loading on DWTS sorption and to simulate a wide concentration range of desalination concentrate from arsenic contaminated groundwater (500 µg/L or 6.67 µM) to industrial wastewater (up to 300 mg/L or 4000 µM), arsenic with various concentrations was spiked into the RO concentrate. Arsenate stock solution was prepared weekly by the dissolution of Na2HAsO4·7H2O (Reagent grade, Fisher Scientific Co., Fair Lawn, NJ, USA) in deionized water.
Arsenic solutions (200 mL) were added in 250 mL polyethylene bottles and 3 mL of sample from each bottle were removed for ICP-MS analysis to measure the initial arsenic concentration before adding DWTS. Different amounts of DWTS were added into each bottle to obtain the sorbent dosages from 1 g solids per liter solution (g/L) to 40 g/L. After adjusting the samples to the desired pH using small amounts of HCl or NaOH solution, the bottles were shaken in a shaker for 24 h to reach sorption equilibrium (Model 3500, VWR, Radnor, PA, USA). Supernatants were taken and analyzed to determine the arsenic sorption under various operating conditions.
Throughout the study, all RO concentrate and treated samples were diluted to levels suitable for analysis using each analytical instrument. Sample collection and handling followed the guidelines in Section 1060 of Standard Methods [39
]. Water samples were filtered through 0.45 µm cellulose acetate filters (Toyo Roshi Kaisha, Ltd., Tokyo, Japan) when applicable.