Evaluation of the lead removal capacity by the adsorption process of Corbula trigona shell powder: modeling and optimization through reponse surface methodology.

This study is based on the evaluation of the adsorption process using Corbula trigona shell powder to remove lead from aqueous solution in a batch mode. Different analytical techniques, including X-ray diffraction, Fourier transform infrared spectroscopy, and EDS-coupled scanning electron microscopy, were used to characterize the shell powder before and after lead treatment. Regarding the pollutant removal, a Plackett-Burman design (PBD) was first used to determine the influencing factors from the following experimental domain: solution pH (3-9), adsorbent mass (0.1-0.5 g), contact time (40 -240 min), initial pollutant concentration (10 -60 mg L-1), and adsorbent size (100 -200 µm). The respective contributions of the various factors listed above are 31.7%, 30.51%, 25.17%, 12.44%, and 0.18%. As a result, solution pH, adsorbent mass, contact time, and initial pollutant concentration were selected to optimize the lead removal process using the composite central plan. The optimal lead removal conditions were 99.028% by setting the solution pH to 4.5, initial lead concentration to 47 mg L-1, contact time to 125 min, and adsorbent mass to 0.2 g. In addition, it was found that the composite central plan could be a reliable statistical tool to model and determine the optimal conditions.

Bivalve shells (Corbula trigona) as a new adsorbent for the defluoridation of groundwater by adsorption-precipitation.

Defluoridation of groundwater was performed in a batch reactor using bivalve shell powder (BSP) as adsorbent. The physicochemical characteristics of BSP, studied by Fourier Transform Infrared, X-ray Diffraction and Inductively Coupled Plasma-Optical Emission Spectrometry after dissolution, have shown that BSP was mainly composed of crystalline CaCO3 (∼97.8%). The effects of pH, initial fluoride concentration, adsorbent dose and contact time on the adsorption capacity of BSP were investigated. For an initial fluoride concentration of 2.2 mg/L and with 16 g/L of BSP, after 8 hours of treatment, 27.3% were eliminated at pH 7.5 versus 68% at pH 3, highlighting the efficiency of the adsorption process. The difference in adsorption capacity as a function of pH was correlated to the pHpzc of the BSP, which was equal to 8.2. Thus, at pH below pHpzc, electrostatic attraction between the fluoride anions and the positively charged adsorbent could justify the adsorption mechanism. Fittings of experimental data have evidenced that the adsorption kinetics were of pseudo-second order whereas the adsorption isotherms were of Langmuir type. The chemical precipitation of calcium fluoride was also revealed to occur upon release of Ca2+ from partial dissolution of CaCO3 in acidic conditions.

Are Presence/Absence Microbial Tests Appropriate for Monitoring Large Urban Water Supplies in Sub-Saharan Africa?

Screening for fecal contamination via microbial water quality monitoring is a critical component of safe drinking water provision and public health protection. Achieving adequate levels of microbial water quality testing, however, is a challenge in resource-limited settings. One strategy for addressing this challenge is to improve the efficiency of monitoring programs. In African countries, quantitative microbial testing methods are commonly used to monitor chlorinated piped water systems. However, presence/absence (P/A) tests may provide an appropriate alternative for water supplies that generally show negative fecal contamination results. This study compares 1048 water quality test results for samples collected from five African urban water systems. The operators of the systems conducted parallel tests on the 1048 samples using their standard quantitative methods (e.g., most probable number or membrane filtration) and the Colitag™ method in P/A format. Combined data demonstrates agreement rates of 97.9% (1024/1046) for detecting total coliforms and 97.8% (1025/1048) for detecting E. coli. We conclude that the P/A test offers advantages as a simpler and similarly sensitive measure of potential fecal contamination for large, urban chlorinated water systems. P/A tests may also offer a cost-effective alternative to quantitative methods, as they are quicker to perform and require less laboratory equipment.