Assessment of compost as a dual-performance bioadsorbent for removing heavy metals and textile dyes: an overview

As is well know, many bioadsorbents have been evaluated for removing toxic heavy metals, dyes, and other contaminants. Composting and vermicomposting have been recommended for recycling organic wastes for a more sustainable environment. In this regard, compost has been examined for sequestering heavy metals and different classes of dyes from wastewater. In this review, the application of compost as a dual bioadsorbent for heavy metals and dyes will be critically discussed. Searching literature revealed that 12 composts manifested a promising retention for diverse classes of dyes, including basic, direct, acid, and reactive. Meanwhile, 24 composts also showed a promising adsorption for diverse toxic metals, including Pb, Cu, Zn, Cd, Ni, and Cr. The overall performance of composts for removing diverse pollutants is assessed through the following requirements: maximum uptake capacity, adsorbent dosage, equilibrium time, working under flow conditions, recycling, and production cost. A comprehensive analysis of adsorption parameters revealed that compost is effective for dyes with an average uptake capacity of 15.0-206.0 mg/g, an average dose of 10.4 g/L, and an average contact time of 181.0 min. For metals, compost is also promising, with an average uptake capacity of 16.8-107.6 mg/g, an average dose of 6.8 g/L, and an average contact time of 119.0 min. The main drawback of utilizing compost as an adsorbent is the high dosage, 6.8-10.4 g/L, which could limit its practicality in large-scale treatment systems. Compost derived from pine bark was excellent for removing Basic Blue 9 with an uptake capacity of 876 mg/g. Compost derived from tree branches and vegetables was active for removing weakly interacting anionic Direct Blue 151 dye with a retention value of 118 mg/g. With a retention value of 170 mg/g, compost derived from mushrooms was effective for removing reactive levafix. For toxic metals, composts derived from sewage sludge and organic wastes were active for removing Pb, Ni, and Cd with retention values of 357, 172, and 85 mg/g, respectively. The reported characterization tests, specific surface area, cation exchange capacity, pH of point of zero charge, and spectral analysis indicated that retention of metals and dyes is based on complex chemical forces including ion exchange, hydrophobic-hydrophobic forces, and H-bonding. Unfortunately, competitive uptake of pollutants, recycling, and working under real wastewater conditions do not receive high attention in the published research, which may not permit a comprehensive assessment of compost as an adsorbent. In this regard, more attention should be paid to examining the recyclability of compost and its workability under flow conditions.