Developing a slow-release permanganate-persulfate composite gel for the in-situ treatment of sulfonamides in groundwater plumes.

A slow-release permanganate (PM) and persulfate (PS) composite gel (P-PGSR) was developed to overcome the blocking effect of amorphous MnO2 generated from PM and the futility of PS without activator in-situ chemical oxidation (ISCO) of sulfonamides (SAs) in groundwater plumes. Results showed that the release longevity of PM and PS from P-PGSR was lengthened 3-fold more than from powder in a column experiment. Low P-PGSR dosage, permeability of the porous medium and ionic strength facilitated the release efficiencies of PM and PS, which was not fully consistent with the degradation efficiencies of SAs. Approximately 99.1% sulfathiazole (STZ) at pH 6, and 83.9% sulfadiazine (SDZ), 92.4% sulfachloropyridazine (SCP) and 92.4% sulfamethoxazole (SMX) at pH 5, were degraded within 24 min by 1.25 g P-PGSR at 2.0 mL min-1 flow rate using 0.01 M NaCl as background solution. Co-existing 0.5 M anions inhibited the degradation of SAs. The addition of 5 mg L-1 fulvic acid enhanced the degradation of SDZ and SCP, but reduced the degradation of STZ and SMX. Increasing ration of PS and PM from 0 to 5 accelerated the kobs of SDZ, STZ, SCP and SMX by 5.5, 1.3, 18.1 and 5.6 folds, respectively, proving that P-PGSR is a novel self-catalytic oxidant to improve the remediation efficiency of ISCO. Radical quenching experiments illustrated that ∙OH was the primarily reactive species for the degradation of STZ and SMX, while SO4∙- played an important role in the degradation of SDZ, STZ and SCP. The intermediate products of SDZ, STZ and SMX indicated that aniline moiety oxidation, hydroxylation, deamination, and smiles-type rearrangement to produce SO2 extrusion were involved in the degradation pathways of SAs by treatment with P-PGSR.