Adsorption Capacities of Iron Hydroxide for Arsenate and Arsenite Removal from Water by Chemical Coagulation: Kinetics, Thermodynamics and Equilibrium Studies

Arsenic in groundwater of the Poyang Lake area (China): aqueous species and health risk assessment

Arsenic is a pervasive pollutant in groundwater, affecting more than 100 million people in 50 countries, including China. Toxicological analysis of As is complicated because As exists in the environment in a variety of forms and redox states. Here, a thermodynamic equilibrium model was used to calculate As speciation, investigate pathways of As accumulation and assess the risk of adverse health effects from oral ingestion of dissolved As from shallow groundwater in the Poyang Lake area (China). The accumulation of As, Fe, and NH₄⁺ in the studied shallow groundwater was found to be the result of the dissolution of As-containing Fe, and probably Mn, (oxyhydr)oxides under reducing conditions due to excess influx of organic matter into the shallow aquifer. Modeling showed that As(III), which is more toxic than As(V), predominated at nearly all sampling sites, regardless of redox conditions. Arsenic tends to accumulate in the highest concentrations as neutral species (As(OH)₃⁰, HAsO₂⁰) under Eh < 50 mV. In the lower reaches of the Ganjiang and Xiushui Rivers, an increased non-carcinogenic risk from oral ingestion of As from drinking water was observed. The elevated cancer risk was found to be present throughout the study area. The lower reaches of the Ganjiang and Xiushui Rivers that have been shown to have the highest risk of both non-carcinogenic and carcinogenic adverse health effects are associated with more toxic As(III) species. Given the As speciation and risk profile, it is recommended to introduce strategies to alter redox conditions in shallow groundwater by adopting safer irrigation practices and managing fertilizer applications to avoid the buildup of high As concentrations associated with adverse health effects.

Arsenic removal from aqueous solution: A comprehensive synthesis with meta-data

Removal of arsenic from drinking water is one of the most important global concerns. Among the various techniques, adsorptive removal of arsenic is considered as a viable most effective method. However, limited attention is given to understand the overall relative sorption capacity of different sorbents (e.g., biocomposite, biochar and nano-composite etc.) since various factors influence the sorption capacity. The aim of this study is to assess the effectiveness of various adsorbents with quantitative estimation (Langmuir adsorption maxima, Qmax) as well as to evaluate the influence of experimental conditions on the achievement of maximum adsorption. A number of analyses including meta-analysis, analysis of variance (ANOVA), scientometric and regression were performed. The results revealed that among the sorbents, nanoparticles show the greatest sorption capacity while pre-doped biochar performed the best among different biochars. Average across all sorbents, As (V) removal efficacy was higher than As (III). As expected, a high point of zero charge (PZC) and higher positive surface charge favored adsorption. The relative contribution of different mechanisms was also discussed. Our scientometric analyses revealed that, research should focus on the development of low-cost adsorbents and increase their reusability, safe disposal of adsorbed arsenic. Altogether, our findings provide a molecular understanding of arsenic sorption to different sorbents with implications for tailoring a good sorbent for arsenic removal from drinking water.

Kinetics and Thermodynamics of Adsorption for Aromatic Hydrocarbon Model Systems via a Coagulation Process with a Ferric Sulfate-Lime Softening System

The adsorption mechanisms for model hydrocarbons, 4-nitrophenol (PNP), and naphthalene were studied in a coagulation-based process using a ferric sulfate-lime softening system. Kinetic and thermodynamic adsorption parameters for this system were obtained under variable ionic strength and temperature. An in situ method was used to investigate kinetic adsorption profiles for PNP and naphthalene, where a pseudo-first order kinetic model adequately described the process. Thermodynamic parameters for the coagulation of PNP and naphthalene reveal an endothermic and spontaneous process. River water was compared against lab water samples at optimized conditions, where the results reveal that ions in the river water decrease the removal efficiency (RE; %) for PNP (RE = 28 to 20.3%) and naphthalene (RE = 89.0 to 80.2%). An aluminum sulfate (alum) coagulant was compared against the ferric system. The removal of PNP with alum decreased from RE = 20.5% in lab water and to RE = 16.8% in river water. Naphthalene removal decreased from RE = 89.0% with ferric sulfate to RE = 83.2% with alum in lab water and from RE = 80.2% for the ferric system to RE = 75.1% for alum in river water. Optical microscopy and dynamic light scattering of isolated flocs corroborated the role of ions in river water, according to variable RE and floc size distribution.