Wastewater generation and treatment is an ever-increasing concern in the current century due to increased urbanization and industrialization. To tackle the situation of increasing environmental hazards, numerous wastewater treatment approaches are used—i.e., physical, chemical, and biological (primary to tertiary treatment) methods. Various treatment techniques being used have the risks of producing secondary pollutants. The most promising technique is the use of different materials as adsorbents that have a higher efficacy in treating wastewater, with a minimal production of secondary pollutants. Biosorption is a key process that is highly efficient and cost-effective. This method majorly uses the adsorption process/mechanism for toxicant removal from wastewater. This review elaborates the major agricultural and non-agricultural materials-based sorbents that have been used with their possible mechanisms of pollutant removal. Moreover, this creates a better understanding of how the efficacy of these sorbents can be enhanced by modification or treatments with other substances. This review also explains the re-usability and mechanisms of the used adsorbents and/or their disposal in a safe and environmentally friendly way, along with highlighting the major research gaps and potential future research directions. Additionally, the cost benefit ratio of adsorbents is elucidated.

Chromium (Cr)-contaminated tannery wastewater is a major environmental concern, especially in developing countries, such as Pakistan, due to its use for crop irrigation, resulting in food-chain contamination and health issues. In this study, we explored the distribution, speciation, hydrogeochemical behavior and environmental risks of Cr in tannery wastewater collected from various tanneries of Kasur district in Punjab, Pakistan. Tannery wastewater samples were taken during the summer (TWW-summer; n = 82) and winter (TWW-winter; n = 82) seasons. The results showed that high Cr concentration was observed in TWW-winter (mean: 49 ± 32 mg L⁻¹) compared to TWW-summer (mean: 15 ± 21 mg L⁻¹). In TWW-summer and TWW-winter samples, the Cr concentration exceeded the National Environmental Quality Standard (1 mg L⁻¹), with the total Cr ranging from 2.8 to 125 mg L⁻¹. Hexavalent Cr (Cr(VI)) and trivalent Cr (Cr(III)) concentrations spanned 2.7 to 2.9 and 12.4 to 46 mg L⁻¹, respectively. The Piper plot showed that hydrogeochemistry of wastewater was dominated by Ca-Mg-SO₄ and Ca-Mg-Cl type water, and geochemical modeling indicated that the presence of Cr-iron (Fe)-bearing mineral phases—notably, FeCr₂O_4, MgCr₂O₄ and Cr(OH)₃) may control the fate of Cr in the tannery wastewater. Environmental risk assessment modeling categorized the tannery wastewater as the ‘worst quality’, which is not fit for use in crop irrigation without treatment. This study highlights that immediate monitoring, remediation and mitigation strategies are required to reduce the risk of Cr exposure from tannery wastewater in many areas of Pakistan.

Water scarcity is one of the key global challenges affecting food safety, food security, and human health. Constructed wetlands (CWs) provide a sustainable tool to remediate wastewater. Here we explored the potential of vertical flow-CWs (VF-CWs) vegetated with ten indigenous wetland plant species to treat chromium (Cr)-contaminated water. The wetland plants were vegetated to develop VF-CWs to treat Cr-contaminated water in a batch mode. Results revealed that the Cr removal potential of VF-CWs vegetated with different wetland plants ranged from 47% to 92% at low (15 mg L⁻¹) Cr levels and 36% to 92% at high (30 mg L⁻¹) Cr levels, with the maximum (92%) Cr removal exhibited by VF-CWs vegetated with Leptochloa fusca. Hexavalent Cr (Cr(VI)) was reduced to trivalent Cr (Cr(III)) in treated water (96–99 %) of all VF-CWs. All the wetland plants accumulated Cr in the shoot (1.9–34 mg kg⁻¹ dry weight (DW)), although Cr content was higher in the roots (74–698 mg kg⁻¹ DW) than in the shoots. Brachiaria mutica showed the highest Cr accumulation in the roots and shoots (698 and 45 mg kg⁻¹ DW, respectively), followed by Leptochloa fusca. The high Cr level significantly (p < 0.05) decreased the stress tolerance index (STI) percentage of the plant species. Our data provide strong evidence to support the application of VF-CWs vegetated with different indigenous wetland plants as a sustainable Cr-contaminated water treatment technology such as tannery wastewater.

In this study, different metal hydroxyapatite (HAP) nanoparticles were synthesized at a laboratory scale. All the methods used were very economical because all reagents used in this work were cheap and easily available in laboratories. The synthesized products were characterized by X-ray diffractometer (XRD), Brunauer–Emmett–Teller and Barrett–Joyner–Halenda, scanning electron microscopy, and Fourier transform infrared spectroscopy analysis to confirm the synthesis of respective products by ensuring the presence of phosphate and hydroxyl functional groups. These metal HAP nanoparticles were applied in water treatment applications as adsorbents for the elimination of turquoise blue dye from the aqueous solution. Batch experiments were performed, and all effective parameters were optimized. Their optimized values were as follows: pH = 8, dosage of adsorbent = 0.05 g, contact time = 75 min, temperature = 30 °C, and dye concentration = 75 ppm. The order of adsorption capacity of four different metal HAP products was determined as calcium-HAP (Ca-HAP) > lead-HAP (Pb-HAP) > strontium-HAP (Sr-HAP) > barium-HAP (Ba-HAP). Ca-HAP, Ba-HAP, Sr-HAP, and Pb-HAP have the highest concentration of 0.5 N, giving 54–69.29% desorption. Moreover, the effect of surfactants and electrolytes was also studied. Langmuir isotherm and pseudo-second-order kinetic model were best fitted for turquoise blue dye adsorption. The reaction was exothermic and spontaneous in nature.