Applicability of agricultural waste and by-products for adsorptive removal of heavy metals from wastewater

•Ability of AWBs for heavy metal detoxification was evaluated.•Influential factors on heavy metal biosorption were presented.•Insights of binding mechanism were revealed and essential tools were introduced.•Merits and demerits of pretreatment methods for better biosorbents were highlighted.•Recommendation to use AWBs as green and cost-effective biosorbents was made. This critical review discusses the potential use of agricultural waste based biosorbents (AWBs) for sequestering heavy metals in terms of their adsorption capacities, binding mechanisms, operating factors and pretreatment methods. The literature survey indicates that AWBs have shown equal or even greater adsorption capacities compared to conventional adsorbents. Thanks to modern molecular biotechnologies, the roles of functional groups in biosorption process are better understood. Of process factors, pH appears to be the most influential. In most cases, chemical pretreatments bring about an obvious improvement in metal uptake capacity. However, there are still several gaps, which require further investigation, such as (i) searching for novel, multi-function AWBs, (ii) developing cost-effective modification methods and (iii) assessing AWBs under multi-metal and real wastewater systems. Once these challenges are settled, the replacement of traditional adsorbents by AWBs in decontaminating heavy metals from wastewater can be expected in the future.