Isolation of Arsenic and Sodium Hydroxide Recovery Using Nanofiltration for a Zero-waste Arsenic Remediation Scheme

Abstract

Groundwater contamination by arsenic poses a significant global health concern, with even trace concentrations leading to severe health issues, including various cancers and cardiovascular diseases. Conventional arsenic remediation technologies often generate arsenic laden waste streams, causing secondary environmental pollution. To address this challenge, a Zero-waste arsenic remediation scheme is developed at the Sustainable Separation Solutions(S3) laboratory, IISc. The scheme consists of three parts: Part 1 is the adsorption- desorption system where the arsenic-contaminated water is passed through the adsorbent bed, the arsenic get adsorbed on the adsorbents and arsenic-free water is obtained. Once the breakthrough occurs, the adsorbent bed is then regenerated using an alkaline NaOH solution. During the regeneration arsenic-rich NaOH solution is produced. This arsenic-rich NaOH solution is then fed to the second part of the scheme, which is a membrane unit where the membrane will separate arsenic and NaOH into two different streams: an arsenic-rich retentate stream and a NaOH-rich permeate stream. This NaOH permeate stream will then be reused for the next cycle of regeneration, hence achieving circularity within the system. The arsenic-rich retentate stream is then fed to the third part of the scheme, which is a biodigester unit, where the arsenic-rich stream is mixed with the cow dung, and the microbes present in the cow dung will convert this toxic inorganic arsenic into less toxic organic arsenic. The current study focuses on part 2 of the scheme, where it is desirable to separate the arsenic and NaOH into two different streams: an arsenic-rich retentate stream and a NaOH-rich permeate stream. Experiments were carried out using commercially available membranes to check the performance of the membrane in separating the arsenic and NaOH. The experimental results showed that the HydraCore70pHT membrane, which is a high pH resistant membrane, has an arsenic rejection of 80% and NaOH rejection of 26% at a pH of 13, at 35 bar transmembrane pressure, which is quite favourable for the scheme. To gain mechanistic insights into solute transport, the experimental data were fitted with the Speigler-Kedem (SK) model and the Donnan Steric Pore Model with Dielectric Exclusion (DSPM-DE) model to get insight into the transport of arsenic and NaOH through the membrane. A system-scale model was developed, integrating the transport equations, experimental data, and rigorous mass balance for each species to calculate the mass recovery of each species in the retentate and permeate streams. System-scale modeling results showed that employing six membrane modules could achieve approximately 80% arsenic mass recovery in the retentate, over 65% NaOH mass recovery in the permeate, and an overall water recovery of 84% in the permeate. These findings demonstrate the promising potential of the nanofiltration membrane unit for effectively separating arsenic and NaOH within the proposed zero-waste remediation scheme, suggesting its feasibility for community-scale implementation.