Trace Element Removal in Distributed Drinking Water Treatment Systems by Cathodic H₂O₂ Production and UV Photolysis

Abstract

As water scarcity intensifies, point-of-use and point-of-entry treatment may provide a means of exploiting locally available water resources that are currently considered to be unsafe for human consumption. Among the different classes of drinking water contaminants, toxic trace elements (e.g., arsenic and lead) pose substantial operational challenges for distributed drinking water treatment systems. Removal of toxic trace elements via adsorption onto iron oxides is an inexpensive and robust treatment method; however, the presence of metal-complexing ligands associated with natural organic matter (NOM) often prevents the formation of iron precipitates at the relatively low concentrations of dissolved iron typically present in natural water sources, thereby requiring the addition of iron which complicates the treatment process and results in a need to dispose of relatively large amounts of accumulated solids. A point-of-use treatment device consisting of a cathodic cell that produced hydrogen peroxide (H2O2) followed by an ultraviolet (UV) irradiation chamber was used to decrease colloid stabilization and metal-complexing capacity of NOM present in groundwater. Exposure to UV light altered NOM, converting ∼6 μM of iron oxides into settable forms that removed between 0.5 and 1 μM of arsenic (As), lead (Pb), and copper (Cu) from solution via adsorption. After treatment, changes in NOM consistent with the loss of iron-complexing carboxylate ligands were observed, including decreases in UV absorbance and shifts in the molecular composition of NOM to higher H/C and lower O/C ratios. Chronoamperometric experiments conducted in synthetic groundwater revealed that the presence of Ca2+ and Mg2+ inhibited intramolecular charge-transfer within photoexcited NOM, leading to substantially increased removal of iron and trace elements.

Molecular-Level Transformation of Dissolved Organic Matter during Oxidation by Ozone and Hydroxyl Radical

“…For example, 88–186 −CO 2 formulas are observed in UMRNOM exposed to both O 3 and • OH, whereas only 18–57 −CO 2 formulas are observed in UMRNOM in the presence of the • OH scavenger (Figure f and Figure S23 and Table S6). This agrees with the ability of • OH to react with DOM via decarboxylation. ,, The number of decarboxylated formulas in SRNOM is similar with and without the scavenger, as well as at pH 11. While this result does not follow the trend observed in UMRNOM, the number of −CO 2 formulas in SRNOM samples is low (i.e., 15–31), suggesting that SRNOM is less amenable to reacting by this mechanism.…”

“…This agrees with the ability of • OH to react with DOM via decarboxylation. 63,65,66 The number of decarboxylated formulas in SRNOM are similar with and without the scavenger, as well as at pH 11. While this result does not follow the trend observed in UMRNOM, the number of -CO2 formulas in SRNOM samples is low (i.e., 15 -31), suggesting that SRNOM is less amenable to reacting by this mechanism.…”

Electrocatalytic Reduction of Nitrate Using Magnéli Phase TiO2 Reactive Electrochemical Membranes Doped with Pd-Based Catalysts

“…However, only RO treatment can accomplish this task. Due to the short-comings of RO treatment mentioned above, other approaches have been investigated, and electrochemical technologies are being researched as POU treatment devices. − Electrochemical technologies can efficiently oxidize various organic contaminants and reduce NO 3 – . However, until recently mass transport limitations in electrochemical cells have prevented the possibility of single pass electrochemical treatment .…”

Graphitic carbon nitride photocatalysis: the hydroperoxyl radical role revealed by kinetic modelling

“…This phenomenon (i.e. decline of H 2 O 2 ) was observed in different studies, being ascribed to different theories: (i) H 2 O 2 acts as a photochemical oxidation agent of the parent molecule and its intermediates; [70][71][72][73] (ii) H 2 O 2 can react directly with the photoactivated exciton pairs, through redox reactions, generating ROS; 14,74 (iii) H 2 O 2 is reduced by…”