Methods of extracting valuable minerals from seawater and seawater brines generated in desalination plants are critically reviewed in this paper.

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Membrane distillation (MD) was evaluated as a treatment option of wastewater reverse osmosis concentrate (WWROC) discharged from wastewater reclamation plants (WRPs). A direct contact MD (DCMD), at obtaining 85% water recovery of WWROC showed only 13-15% flux decline and produced good quality permeate (10-15 µS/cm, 99% ion rejection) at moderate feed temperature of 55 ºC. Prevalent calcium carbonate (CaCO 3 ) deposition on the MD membrane occurred in treating WWROC at elevated concentrations. The combination of low salinity and loose CaCO 3 adhesion on the membrane did not significantly contribute to DCMD flux decline. Meanwhile, high organic content in WWROC (58-60 mg/L) resulted in a significant membrane hydrophobicity reduction (70% lower water contact angle than virgin membrane) attributed to low molecular weight organic adhesion onto the MD membrane. Granular activated carbon (GAC) pretreatment helped in reducing organic contents of WWROC by 46-50%, and adsorbed a range of hydrophobic and hydrophilic micropollutants. This ensured high quality water production by MD (micropollutants-free) and 2 enhanced its reuse potential. The MD concentrated WWROC was suitable for selective ion precipitation, promising a near zero liquid discharge in WRPs.

An attempt has been made in this review to provide some insights into the possible adsorption mechanisms of hexavalent chromium onto layered double hydroxides-based adsorbents by critically examining the past and present literature. Layered double hydroxides (LDH) nanomaterials are typical dual-electronic adsorbents because they exhibit positively charged external surfaces and abundant interlayer anions. A high positive zeta potential value indicates that LDH has a high affinity to Cr(VI) anions in solution through electrostatic attraction. The host interlayer anions (i.e., Cl -, NO3 -, SO4 2-, and CO3 2-) provide a high anion exchange capacity (53-520 meq/100g) which is expected to have an excellent exchangeable capacity to Cr(VI) oxyanions in water. Regarding the adsorptioncoupled reduction mechanism, when Cr(VI) anions make contact with the electron-donor groups in the LDH, they are partly reduced to Cr(III) cations. The reduced Cr(III) cations are then adsorbed by LDH via numerous interactions, such as isomorphic substitution and complexation. Nonetheless, the adsorption-coupled reduction mechanism is greatly dependent on: (1) the nature of divalent and trivalent salts utilized in LDH preparation, and (2) the types of interlayer anions (i.e., guest intercalated organic anions). The low Brunauer-Emmett-Teller specific surface area of LDH (1.80-179 m 2 /g) suggests that pore filling played an insignificant role in Cr(VI) adsorption. The Langmuir maximum adsorption capacity of LDH (Q o max) toward Cr(VI) was significantly affected by the used inorganic salt natures and synthetic methods of LDH. The Q o max values range from 16.3 mg/g to 726 mg/g. Almost all adsorption processes of Cr(VI) by LDH-based adsorbent occur spontaneously (∆G° <0) and endothermically (∆H° >0) and increase the randomness (∆S° >0) in the system. Thus LDH has much potential as a material that can effectively remove anion pollutants, especially Cr(VI) anions in industrial wastewater.